t1501: demonstrate NULL pointer access with invalid GIT_WORK_TREE

When GIT_WORK_TREE does not specify a valid path, we should error
out, instead of crashing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1501-work-tree.sh

@@ -423,4 +423,12 @@ test_expect_success '$GIT_WORK_TREE overrides $GIT_DIR/common' '
 	)
 '
 
+test_expect_failure 'error out gracefully on invalid $GIT_WORK_TREE' '
+	(
+		GIT_WORK_TREE=/.invalid/work/tree &&
+		export GIT_WORK_TREE &&
+		test_expect_code 128 git rev-parse
+	)
+'
+
 test_done