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