CBlock::WriteToDisk() properly checks ftell(3) for error return

Rather than storing ftell(3)'s return value -- a long -- in an
unsigned int, we store and check a properly typed temp.  Then, assured a
non-negative value, we store in nBlockPosRet.

src/main.h

@@ -944,9 +944,10 @@ public:
         fileout << FLATDATA(pchMessageStart) << nSize;
 
         // Write block
-        nBlockPosRet = ftell(fileout);
+        long fileOutPos = ftell(fileout);
-        if (nBlockPosRet == -1)
+        if (fileOutPos < 0)
             return error("CBlock::WriteToDisk() : ftell failed");
+        nBlockPosRet = fileOutPos;
         fileout << *this;
 
         // Flush stdio buffers and commit to disk before returning