stash show: fix segfault with --{include,only}-untracked

When `git stash show --include-untracked` or
`git stash show --only-untracked` is run on a stash that doesn't include
an untracked entry, a segfault occurs. This happens because we do not
check whether the untracked entry is actually present and just attempt
to blindly dereference it.

Ensure that the untracked entry is present before actually attempting to
dereference it.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -900,10 +900,14 @@ static int show_stash(int argc, const char **argv, const char *prefix)
 		diff_tree_oid(&info.b_commit, &info.w_commit, "", &rev.diffopt);
 		break;
 	case UNTRACKED_ONLY:
+		if (info.has_u)
 			diff_root_tree_oid(&info.u_tree, "", &rev.diffopt);
 		break;
 	case UNTRACKED_INCLUDE:
+		if (info.has_u)
 			diff_include_untracked(&info, &rev.diffopt);
+		else
+			diff_tree_oid(&info.b_commit, &info.w_commit, "", &rev.diffopt);
 		break;
 	}
 	log_tree_diff_flush(&rev);

t/t3905-stash-include-untracked.sh

@@ -405,4 +405,19 @@ test_expect_success 'stash show --include-untracked errors on duplicate files' '
 	test_i18ngrep "worktree and untracked commit have duplicate entries: tracked" err
 '
 
+test_expect_success 'stash show --{include,only}-untracked on stashes without untracked entries' '
+	git reset --hard &&
+	git clean -xf &&
+	>tracked &&
+	git add tracked &&
+	git stash &&
+
+	git stash show >expect &&
+	git stash show --include-untracked >actual &&
+	test_cmp expect actual &&
+
+	git stash show --only-untracked >actual &&
+	test_must_be_empty actual
+'
+
 test_done