apply: check git diffs for missing old filenames

2c93286a (fix "git apply --index ..." not to deref NULL) added a check
for git patches missing a +++ line, preventing a segfault.  Check for
missing --- lines as well, and add a test for each case.

Found by Vegard Nossum using AFL.

Original-patch-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

apply.c

@@ -1585,7 +1585,8 @@ static int find_header(struct apply_state *state,
 				patch->old_name = xstrdup(patch->def_name);
 				patch->new_name = xstrdup(patch->def_name);
 			}
-			if (!patch->is_delete && !patch->new_name) {
+			if ((!patch->new_name && !patch->is_delete) ||
+			    (!patch->old_name && !patch->is_new)) {
 				error(_("git diff header lacks filename information "
 					     "(line %d)"), state->linenr);
 				return -128;

t/t4133-apply-filenames.sh

@@ -35,4 +35,28 @@ test_expect_success 'apply diff with inconsistent filenames in headers' '
 	test_i18ngrep "inconsistent old filename" err
 '
 
+test_expect_success 'apply diff with new filename missing from headers' '
+	cat >missing_new_filename.diff <<-\EOF &&
+	diff --git a/f b/f
+	index 0000000..d00491f
+	--- a/f
+	@@ -0,0 +1 @@
+	+1
+	EOF
+	test_must_fail git apply missing_new_filename.diff 2>err &&
+	test_i18ngrep "lacks filename information" err
+'
+
+test_expect_success 'apply diff with old filename missing from headers' '
+	cat >missing_old_filename.diff <<-\EOF &&
+	diff --git a/f b/f
+	index d00491f..0000000
+	+++ b/f
+	@@ -1 +0,0 @@
+	-1
+	EOF
+	test_must_fail git apply missing_old_filename.diff 2>err &&
+	test_i18ngrep "lacks filename information" err
+'
+
 test_done