git log: avoid segfault with --all-match

Avoid a segfault when the command

	git log --all-match

was issued, by ignoring the option.

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -192,6 +192,7 @@ void compile_grep_patterns(struct grep_opt *opt)
 	 * A classic recursive descent parser would do.
 	 */
 	p = opt->pattern_list;
+	if (p)
 		opt->pattern_expression = compile_pattern_expr(&p);
 	if (p)
 		die("incomplete pattern expression: %s", p->pattern);