fetch: avoid unnecessary work when there is no current branch

As pointed out by CodeQL, `branch_get()` may return `NULL`, in which
case `branch_has_merge_config()` would return early, but we can even
avoid enumerating the refs prefixes in that case, saving even more CPU
cycles.

Technically, we should enclose these two statements in an `if (branch)
{...}` block, but the indentation is already quite deep, therefore I
refrained from doing that.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

builtin/fetch.c

@@ -1728,7 +1728,7 @@ static int do_fetch(struct transport *transport,
 			if (transport->remote->follow_remote_head != FOLLOW_REMOTE_NEVER)
 				do_set_head = 1;
 		}
-		if (branch_has_merge_config(branch) &&
+		if (branch && branch_has_merge_config(branch) &&
 		    !strcmp(branch->remote_name, transport->remote->name)) {
 			int i;
 			for (i = 0; i < branch->merge_nr; i++) {