[PATCH v3] diff: make diff_free_filespec_data accept NULL

[Jinoh Kang <luke1337@theori.io>]

Commit 3aef54e8b8 ("diff: munmap() file contents before running external
diff") introduced calls to diff_free_filespec_data in
run_external_diff, which may pass NULL pointers.

Fix this and prevent any such bugs in the future by making
`diff_free_filespec_data(NULL)` a no-op.

Fixes: 3aef54e8b8 ("diff: munmap() file contents before running external diff")
Signed-off-by: Jinoh Kang <luke1337@theori.io>
---
 diff.c              |  3 +++
 t/t7800-difftool.sh | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/diff.c b/diff.c
index d24f47df99..ace4a1d387 100644
--- a/diff.c
+++ b/diff.c
@@ -4115,6 +4115,9 @@ void diff_free_filespec_blob(struct diff_filespec *s)
 
 void diff_free_filespec_data(struct diff_filespec *s)
 {
+	if (!s)
+		return;
+
 	diff_free_filespec_blob(s);
 	FREE_AND_NULL(s->cnt_data);
 }
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 524f30f7dc..e9391abb54 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -728,6 +728,29 @@ test_expect_success 'add -N and difftool -d' '
 	git difftool --dir-diff --extcmd ls
 '
 
+test_expect_success 'difftool --cached with unmerged files' '
+	test_when_finished git reset --hard &&
+	echo base >file &&
+	git add file &&
+	git commit -m base &&
+	git checkout -B conflict-a &&
+	git checkout -B conflict-b &&
+	git checkout conflict-a &&
+	echo conflict-a >>file &&
+	git add file &&
+	git commit -m conflict-a &&
+	git checkout conflict-b &&
+	echo conflict-b >>file &&
+	git add file &&
+	git commit -m conflict-b &&
+	git checkout master &&
+	git merge conflict-a &&
+	test_must_fail git merge conflict-b &&
+	: >expect &&
+	git difftool --cached --no-prompt >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'outside worktree' '
 	echo 1 >1 &&
 	echo 2 >2 &&
-- 
2.26.2