grep: fix segfault when "git grep '('" is given

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7002-grep.sh

@@ -26,6 +26,10 @@ test_expect_success setup '
 	git commit -m initial
 '
 
+test_expect_success 'grep should not segfault with a bad input' '
+	test_must_fail git grep "("
+'
+
 for H in HEAD ''
 do
 	case "$H" in