log: handle broken HEAD in decoration check

The resolve_ref_unsafe() function may return NULL even with
a REF_ISSYMREF flag if a symref points to a broken ref. As a
result, it's possible for the decoration code's "is this
branch the current HEAD" check to segfault when it passes
the NULL to starts_with().

This is unlikely in practice, since we can only reach this
code if we already resolved HEAD to a matching sha1 earlier.
But it's possible if HEAD racily becomes broken, or if
there's a transient filesystem error.

We can fix this by returning early in the broken case, since
NULL could not possibly match any of our branch names.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

@@ -198,7 +198,7 @@ static const struct name_decoration *current_pointed_by_HEAD(const struct name_d
 
 	/* Now resolve and find the matching current branch */
 	branch_name = resolve_ref_unsafe("HEAD", 0, NULL, &rru_flags);
-	if (!(rru_flags & REF_ISSYMREF))
+	if (!branch_name || !(rru_flags & REF_ISSYMREF))
 		return NULL;
 
 	if (!starts_with(branch_name, "refs/"))