submodule--helper: avoid memory leak when fetching submodules

In c51f8f94e5 (submodule--helper: run update procedures from C,
2021-08-24), we added code that first obtains the default remote, and
then adds that to a `strvec`.

However, we never released the default remote's memory.

Reported by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -2208,6 +2208,7 @@ static int fetch_in_submodule(const char *module_path, int depth, int quiet, str
 		char *hex = oid_to_hex(oid);
 		char *remote = get_default_remote();
 		strvec_pushl(&cp.args, remote, hex, NULL);
+		free(remote);
 	}
 
 	return run_command(&cp);