BUG/MINOR: config: Fix memory leak on config parse listen

This memory leak happens if there is two or more defaults section. When
the default proxy is reinitialized, the structure member containing the
config filename must be freed.

Fix github issue #851.
Should be backported as far as 1.6.

src/cfgparse-listen.c

@@ -479,6 +479,7 @@ int cfg_parse_listen(const char *file, int linenum, char **args, int kwm)
 			goto out;
 		}
 
+		free(defproxy.conf.file);
 		free(defproxy.check_command);
 		free(defproxy.check_path);
 		free(defproxy.cookie_name);