git_checkout_tree options fix

According to the reference the git_checkout_tree and git_checkout_head
functions should accept NULL in the opts field

This was broken since the opts field was dereferenced and thus lead to a
crash.

src/checkout.c

@@ -2699,7 +2699,7 @@ int git_checkout_tree(
 	if ((error = git_repository_index(&index, repo)) < 0)
 		return error;
 
-	if ((opts->checkout_strategy & GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH)) {
+	if (opts && (opts->checkout_strategy & GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH)) {
 		iter_opts.pathlist.count = opts->paths.count;
 		iter_opts.pathlist.strings = opts->paths.strings;
 	}

tests/checkout/tree.c

@@ -1479,3 +1479,7 @@ void test_checkout_tree__baseline_is_empty_when_no_index(void)
 	git_reference_free(head);
 }
 
+void test_checkout_tree__nullopts(void)
+{
+	cl_git_pass(git_checkout_tree(g_repo, NULL, NULL));
+}