ftp: fix memory leak in ftp_done

If after a transfer is complete Curl_GetFTPResponse() returns an error,
curl would not free the ftp->pathalloc block.

Found by torture-testing test 576

Closes curl#6737

lib/ftp.c

@@ -3325,8 +3325,10 @@ static CURLcode ftp_done(struct Curl_easy *data, CURLcode status,
       connclose(conn, "Timeout or similar in FTP DONE operation"); /* close */
     }
 
-    if(result)
+    if(result) {
+      Curl_safefree(ftp->pathalloc);
       return result;
+    }
 
     if(ftpc->dont_check && data->req.maxdownload > 0) {
       /* we have just sent ABOR and there is no reliable way to check if it was