get_url_file_name: Fixed crash on OOM on debug build

This caused a null-pointer dereference which caused a few dozen
torture tests to fail.

src/tool_operhlp.c

@@ -164,6 +164,8 @@ CURLcode get_url_file_name(char **filename, const char *url)
       Curl_safefree(*filename);
       *filename = strdup(buffer); /* clone the buffer */
       curl_free(tdir);
+      if(!*filename)
+        return CURLE_OUT_OF_MEMORY;
     }
   }
 #endif