OpenBlockFile(): cast to eliminate signed/unsigned comparison warning

nFile's null value is -1.  Cast that to unsigned int, to avoid warning.

Additionally, avoid nFile==0 because the first valid value is 1.

src/main.cpp

@@ -1836,7 +1836,7 @@ bool CheckDiskSpace(uint64 nAdditionalBytes)
 
 FILE* OpenBlockFile(unsigned int nFile, unsigned int nBlockPos, const char* pszMode)
 {
-    if (nFile == -1)
+    if ((nFile < 1) || (nFile == (unsigned int) -1))
         return NULL;
     FILE* file = fopen((GetDataDir() / strprintf("blk%04d.dat", nFile)).string().c_str(), pszMode);
     if (!file)