multi_runsingle: fix possible memory leak

Coverity CID 1202837. 'newurl' can in fact be allocated even when
Curl_retry_request() returns failure so free it if need be.

lib/multi.c

@@ -1365,6 +1365,7 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
           else {
             /* Have error handler disconnect conn if we can't retry */
             disconnect_conn = TRUE;
+            free(newurl);
           }
         }
         else {