ftp: fix Codacy/cppcheck warning about null pointer arithmetic

Increment `bytes` only if it is non-null.

Closes https://github.com/curl/curl/pull/6576

lib/ftp.c

@@ -2482,7 +2482,8 @@ static CURLcode ftp_state_get_resp(struct Curl_easy *data,
           bytes--;
         }
         /* if we have nothing but digits: */
-        if(bytes++) {
+        if(bytes) {
+          ++bytes;
           /* get the number! */
           (void)curlx_strtoofft(bytes, NULL, 0, &size);
         }