Convert to unsigned char before applying bit shift operator (#4664)

* Convert to unsigned char before applying bit shift operator

* convert to const unsigned char inline

* convert to unsigned char inline

include/flatbuffers/util.h

@@ -321,7 +321,7 @@ inline int FromUTF8(const char **in) {
       break;
     }
   }
-  if ((**in << len) & 0x80) return -1;  // Bit after leading 1's must be 0.
+  if ((static_cast<const unsigned char>(**in) << len) & 0x80) return -1;  // Bit after leading 1's must be 0.
   if (!len) return *(*in)++;
   // UTF-8 encoded values with a length are between 2 and 4 bytes.
   if (len < 2 || len > 4) { return -1; }