trailer: fix leaking trailer values

Fix leaking trailer values when replacing the value with a command or
when the token value is empty.

This leak is exposed by t7513, but plugging it does not make the whole
test suite pass.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

trailer.c

@@ -249,17 +249,23 @@ static char *apply_command(struct conf_info *conf, const char *arg)
 static void apply_item_command(struct trailer_item *in_tok, struct arg_item *arg_tok)
 {
 	if (arg_tok->conf.command || arg_tok->conf.cmd) {
-		const char *arg;
+		char *value_to_free = NULL;
+		char *arg;
+
 		if (arg_tok->value && arg_tok->value[0]) {
 			arg = arg_tok->value;
 		} else {
 			if (in_tok && in_tok->value)
 				arg = xstrdup(in_tok->value);
 			else
 				arg = xstrdup("");
+			value_to_free = arg_tok->value;
 		}
+
 		arg_tok->value = apply_command(&arg_tok->conf, arg);
-		free((char *)arg);
+
+		free(value_to_free);
+		free(arg);
 	}
 }