Fix bugs on 'unsigned char' platforms.

In ISO C++, the signedness of 'char' is undefined.  On some platforms (e.g.
ARM), 'char' is an unsigned type, but some of the code relies on 'char' being
signed (as it is on x86).  This is indicated by compiler warnings like this:

 bignum.h: In constructor 'CBigNum::CBigNum(char)':
 bignum.h:81:59: warning: comparison is always true due to limited range of data type [-Wtype-limits]

 util.cpp: In function 'bool IsHex(const string&)':
 util.cpp:427:28: warning: comparison is always false due to limited range of data type [-Wtype-limits]

In particular, IsHex erroneously returned true regardless of the input
characters, as long as the length of the string was a positive multiple of 2.

Note: For testing, it's possible using GCC to force char to be unsigned by
adding the -funsigned-char parameter to xCXXFLAGS.

src/bignum.h

@@ -78,7 +78,8 @@ public:
         BN_clear_free(this);
     }
 
-    CBigNum(char n)             { BN_init(this); if (n >= 0) setulong(n); else setint64(n); }
+    //CBigNum(char n) is not portable.  Use 'signed char' or 'unsigned char'.
+    CBigNum(signed char n)      { BN_init(this); if (n >= 0) setulong(n); else setint64(n); }
     CBigNum(short n)            { BN_init(this); if (n >= 0) setulong(n); else setint64(n); }
     CBigNum(int n)              { BN_init(this); if (n >= 0) setulong(n); else setint64(n); }
     CBigNum(long n)             { BN_init(this); if (n >= 0) setulong(n); else setint64(n); }
@@ -296,7 +297,7 @@ public:
             psz++;
 
         // hex string to bignum
-        static char phexdigit[256] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,1,2,3,4,5,6,7,8,9,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0 };
+        static signed char phexdigit[256] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,1,2,3,4,5,6,7,8,9,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0 };
         *this = 0;
         while (isxdigit(*psz))
         {

src/script.h

@@ -268,7 +268,8 @@ public:
     }
 
 
-    explicit CScript(char b)           { operator<<(b); }
+    //explicit CScript(char b) is not portable.  Use 'signed char' or 'unsigned char'.
+    explicit CScript(signed char b)    { operator<<(b); }
     explicit CScript(short b)          { operator<<(b); }
     explicit CScript(int b)            { operator<<(b); }
     explicit CScript(long b)           { operator<<(b); }
@@ -285,7 +286,8 @@ public:
     explicit CScript(const std::vector<unsigned char>& b) { operator<<(b); }
 
 
-    CScript& operator<<(char b)           { return push_int64(b); }
+    //CScript& operator<<(char b) is not portable.  Use 'signed char' or 'unsigned char'.
+    CScript& operator<<(signed char b)    { return push_int64(b); }
     CScript& operator<<(short b)          { return push_int64(b); }
     CScript& operator<<(int b)            { return push_int64(b); }
     CScript& operator<<(long b)           { return push_int64(b); }

src/uint256.h

@@ -305,7 +305,7 @@ public:
             psz += 2;
 
         // hex string to uint
-        static char phexdigit[256] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,1,2,3,4,5,6,7,8,9,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0 };
+        static unsigned char phexdigit[256] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,1,2,3,4,5,6,7,8,9,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0 };
         const char* pbegin = psz;
         while (phexdigit[(unsigned char)*psz] || *psz == '0')
             psz++;

src/util.cpp

@@ -425,7 +425,7 @@ bool ParseMoney(const char* pszIn, int64& nRet)
 }
 
 
-static char phexdigit[256] =
+static signed char phexdigit[256] =
 { -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
   -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
   -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
@@ -461,12 +461,12 @@ vector<unsigned char> ParseHex(const char* psz)
     {
         while (isspace(*psz))
             psz++;
-        char c = phexdigit[(unsigned char)*psz++];
-        if (c == (char)-1)
+        signed char c = phexdigit[(unsigned char)*psz++];
+        if (c == (signed char)-1)
             break;
         unsigned char n = (c << 4);
         c = phexdigit[(unsigned char)*psz++];
-        if (c == (char)-1)
+        if (c == (signed char)-1)
             break;
         n |= c;
         vch.push_back(n);