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>

grep.c

@@ -54,6 +54,8 @@ static struct grep_expr *compile_pattern_atom(struct grep_pat **list)
 	struct grep_expr *x;
 
 	p = *list;
+	if (!p)
+		return NULL;
 	switch (p->token) {
 	case GREP_PATTERN: /* atom */
 	case GREP_PATTERN_HEAD:
@@ -66,8 +68,6 @@ static struct grep_expr *compile_pattern_atom(struct grep_pat **list)
 	case GREP_OPEN_PAREN:
 		*list = p->next;
 		x = compile_pattern_or(list);
-		if (!x)
-			return NULL;
 		if (!*list || (*list)->token != GREP_CLOSE_PAREN)
 			die("unmatched parenthesis");
 		*list = (*list)->next;
@@ -83,6 +83,8 @@ static struct grep_expr *compile_pattern_not(struct grep_pat **list)
 	struct grep_expr *x;
 
 	p = *list;
+	if (!p)
+		return NULL;
 	switch (p->token) {
 	case GREP_NOT:
 		if (!p->next)
@@ -361,6 +363,8 @@ static int match_expr_eval(struct grep_opt *o,
 {
 	int h = 0;
 
+	if (!x)
+		die("Not a valid grep expression");
 	switch (x->node) {
 	case GREP_NODE_ATOM:
 		h = match_one_pattern(o, x->u.atom, bol, eol, ctx);

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