NUL terminate decoded base64 string

For consistency with encoded strings.

src/string_util.cpp

@@ -362,6 +362,7 @@ unsigned char* s3fs_decode64(const char* input, size_t* plength)
     }
     result[wpos++] = ((parts[2] << 6) & 0xc0) | (parts[3] & 0x3f);
   }
+  result[wpos] = '\0';
   *plength = wpos;
   return result;
 }