show-branch: avoid segfault with --reflog of unborn branch

When no branch is given to the "--reflog" option, we resolve
HEAD to get the default branch. However, if HEAD points to
an unborn branch, resolve_ref returns NULL, and we later
segfault trying to access it.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/show-branch.c

@@ -743,6 +743,8 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			fake_av[1] = NULL;
 			av = fake_av;
 			ac = 1;
+			if (!*av)
+				die("no branches given, and HEAD is not valid");
 		}
 		if (ac != 1)
 			die("--reflog option needs one branch name");