worktree: don't segfault with an absolute pathspec without a work tree

If a command is run with an absolute path as a pathspec inside a bare
repository, e.g. "rev-list HEAD -- /home", the code tried to run strlen()
on NULL, which is the result of get_git_work_tree(), and segfaulted.  It
should just fail instead.

Currently the function returns NULL even inside .git/ in a repository
with a work tree, but that is a separate issue.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

setup.c

@@ -18,9 +18,12 @@ const char *prefix_path(const char *prefix, int len, const char *path)
 	if (normalize_path_copy(sanitized, sanitized))
 		goto error_out;
 	if (is_absolute_path(orig)) {
+		size_t len, total;
 		const char *work_tree = get_git_work_tree();
-		size_t len = strlen(work_tree);
-		size_t total = strlen(sanitized) + 1;
+		if (!work_tree)
+			goto error_out;
+		len = strlen(work_tree);
+		total = strlen(sanitized) + 1;
 		if (strncmp(sanitized, work_tree, len) ||
 		    (sanitized[len] != '\0' && sanitized[len] != '/')) {
 		error_out:

t/t1501-worktree.sh

@@ -174,4 +174,19 @@ test_expect_success 'git grep' '
 	GIT_DIR=../.. GIT_WORK_TREE=.. git grep -l changed | grep dir/tracked)
 '
 
+test_expect_success 'git commit' '
+	(
+		cd repo.git &&
+		GIT_DIR=. GIT_WORK_TREE=work git commit -a -m done
+	)
+'
+
+test_expect_success 'absolute pathspec should fail gracefully' '
+	(
+		cd repo.git || exit 1
+		git config --unset core.worktree
+		test_must_fail git log HEAD -- /home
+	)
+'
+
 test_done