Fix some empty vector references

streams.h has some methods that can be tricked into dereferencing
null pointers or end() iterators. Fix this.

src/streams.h

@@ -248,7 +248,8 @@ public:
 
     void insert(iterator it, std::vector<char>::const_iterator first, std::vector<char>::const_iterator last)
     {
-        assert(last - first >= 0);
+        if (last == first) return;
+        assert(last - first > 0);
         if (it == vch.begin() + nReadPos && (unsigned int)(last - first) <= nReadPos)
         {
             // special case for inserting at the front when there's room
@@ -261,7 +262,8 @@ public:
 
     void insert(iterator it, const char* first, const char* last)
     {
-        assert(last - first >= 0);
+        if (last == first) return;
+        assert(last - first > 0);
         if (it == vch.begin() + nReadPos && (unsigned int)(last - first) <= nReadPos)
         {
             // special case for inserting at the front when there's room
@@ -339,6 +341,8 @@ public:
 
     void read(char* pch, size_t nSize)
     {
+        if (nSize == 0) return;
+
         // Read from the beginning of the buffer
         unsigned int nReadPosNext = nReadPos + nSize;
         if (nReadPosNext >= vch.size())