Merge branch 'jx/do-not-crash-receive-pack-wo-head'

An attempt to delete a ref by pushing into a repositorywhose HEAD
symbolic reference points at an unborn branch that cannot be
created due to ref D/F conflict (e.g. refs/heads/a/b exists, HEAD
points at refs/heads/a) failed.

* jx/do-not-crash-receive-pack-wo-head:
  receive-pack: crash when checking with non-exist HEAD

builtin/receive-pack.c

@@ -933,7 +933,7 @@ 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;