bisect: ensure non-NULL head before using it

When `refs_resolve_ref_unsafe()` is called to resolve HEAD, and returns
NULL (e.g., HEAD does not exist as a proper ref), the code falls back to
`repo_get_oid("HEAD")` to try to resolve the OID directly. If that
succeeds, execution continues with `head` still set to NULL.

Later, that variable is passed to `repo_get_oid()` and `starts_with()`,
both of which would dereference the NULL pointer.

The scenario "`refs_resolve_ref_unsafe()` returns NULL but
`repo_get_oid()` succeeds" can happen when HEAD is a detached bare OID
that the ref backend cannot resolve symbolically (a potential edge case
with the reftable backend) but the OID itself is valid. In this case,
the bisect-start file does not yet exist (this is a fresh "git bisect
start"), so the else branch is taken with the NULL `head`.

Simply assign "HEAD" to `head` as a fallback to address this.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/bisect.c

@@ -811,9 +811,11 @@ static enum bisect_error bisect_start(struct bisect_terms *terms, int argc,
 	 */
 	head = refs_resolve_ref_unsafe(get_main_ref_store(the_repository),
 				       "HEAD", 0, &head_oid, &flags);
-	if (!head)
+	if (!head) {
 		if (repo_get_oid(the_repository, "HEAD", &head_oid))
 			return error(_("bad HEAD - I need a HEAD"));
+		head = "HEAD";
+	}
 
 	/*
 	 * Check if we are bisecting