receive-pack: crash when checking with non-exist HEAD

If HEAD of a repository points to a conflict reference, such as:

* There exist a reference named 'refs/heads/jx/feature1', but HEAD
  points to 'refs/heads/jx', or

* There exist a reference named 'refs/heads/feature', but HEAD points
  to 'refs/heads/feature/bad'.

When we push to delete a reference for this repo, such as:

        git push /path/to/bad-head-repo.git :some/good/reference

The git-receive-pack process will crash.

This is because if HEAD points to a conflict reference, the function
`resolve_refdup("HEAD", ...)` does not return a valid reference name,
but a null buffer.  Later matching the delete reference against the null
buffer will cause git-receive-pack crash.

Signed-off-by: Jiang Xin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jiangxin

builtin/receive-pack.c

@@ -514,7 +514,7 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 			return "deletion prohibited";
 		}
 
-		if (!strcmp(namespaced_name, head_name)) {
+		if (head_name && !strcmp(namespaced_name, head_name)) {
 			switch (deny_delete_current) {
 			case DENY_IGNORE:
 				break;