grep: do not segfault when -f is used

"git grep" would segfault if its -f option was used because it would
try to use an uninitialized strbuf, so initialize the strbuf.

Thanks to Johannes Sixt <j.sixt@viscovery.net> for the help with the
test cases.
Signed-off-by: NMatt Kraai <kraai@ftbfs.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -599,7 +599,7 @@ static int file_callback(const struct option *opt, const char *arg, int unset)
 	struct grep_opt *grep_opt = opt->value;
 	FILE *patterns;
 	int lno = 0;
-	struct strbuf sb;
+	struct strbuf sb = STRBUF_INIT;
 
 	patterns = fopen(arg, "r");
 	if (!patterns)

t/t7002-grep.sh

@@ -164,6 +164,72 @@ test_expect_success 'grep -e A --and --not -e B' '
 	test_cmp expected actual
 '
 
+test_expect_success 'grep -f, non-existent file' '
+	test_must_fail git grep -f patterns
+'
+
+cat >expected <<EOF
+file:foo mmap bar
+file:foo_mmap bar
+file:foo_mmap bar mmap
+file:foo mmap bar_mmap
+file:foo_mmap bar mmap baz
+EOF
+
+cat >pattern <<EOF
+mmap
+EOF
+
+test_expect_success 'grep -f, one pattern' '
+	git grep -f pattern >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+file:foo mmap bar
+file:foo_mmap bar
+file:foo_mmap bar mmap
+file:foo mmap bar_mmap
+file:foo_mmap bar mmap baz
+t/a/v:vvv
+t/v:vvv
+v:vvv
+EOF
+
+cat >patterns <<EOF
+mmap
+vvv
+EOF
+
+test_expect_success 'grep -f, multiple patterns' '
+	git grep -f patterns >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+file:foo mmap bar
+file:foo_mmap bar
+file:foo_mmap bar mmap
+file:foo mmap bar_mmap
+file:foo_mmap bar mmap baz
+t/a/v:vvv
+t/v:vvv
+v:vvv
+EOF
+
+cat >patterns <<EOF
+
+mmap
+
+vvv
+
+EOF
+
+test_expect_success 'grep -f, ignore empty lines' '
+	git grep -f patterns >actual &&
+	test_cmp expected actual
+'
+
 cat >expected <<EOF
 y:y yy
 --