Fixed compilation with -Wsign-conversion on mac

src/core/support/log_posix.c

@@ -64,7 +64,7 @@ void gpr_log(const char *file, int line, gpr_log_severity severity,
   } else {
     message = allocated = gpr_malloc((size_t)ret + 1);
     va_start(args, format);
-    vsnprintf(message, ret + 1, format, args);
+    vsnprintf(message, (size_t)(ret + 1), format, args);
     va_end(args);
   }
   gpr_log_message(file, line, severity, message);