[RFC PATCH 04/15] diff-lib.c: don't dereference NULL in oneway_diff()

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Fix a control flow issue dating back to d1f2d7e8ca6 (Make
run_diff_index() use unpack_trees(), not read_tree(), 2008-01-19)
where we'd assume "tree" must be non-NULL if idx was NULL. As
-fanalyzer shows we'd end up dereferencing "tree" in that case in
ce_path_match():

dir.h:541:41: warning: dereference of NULL ‘ce’ [CWE-476] [-Wanalyzer-null-dereference]
  541 |                               S_ISDIR(ce->ce_mode) || S_ISGITLINK(ce->ce_mode));
      |                                         ^
  ‘oneway_diff’: events 1-2
    |
    |diff-lib.c:493:12:
    |  493 | static int oneway_diff(const struct cache_entry * const *src,
    |      |            ^~~~~~~~~~~
    |      |            |
    |      |            (1) entry to ‘oneway_diff’
    |......
    |  506 |         if (tree == o->df_conflict_entry)
    |      |            ~
    |      |            |
    |      |            (2) following ‘true’ branch...
    |
  ‘oneway_diff’: event 3
    |
    |  507 |                 tree = NULL;
    |      |                      ^
    |      |                      |
    |      |                      (3) ...to here
    |
  ‘oneway_diff’: events 4-8
    |
    |  507 |                 tree = NULL;
    |      |                      ^
    |      |                      |
    |      |                      (4) ‘tree’ is NULL
    |  508 |
    |  509 |         if (ce_path_match(revs->diffopt.repo->index,
    |      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |             |
    |      |             (5) following ‘false’ branch (when ‘idx’ is NULL)...
    |      |             (6) ...to here
    |      |             (7) ‘tree’ is NULL
    |      |             (8) calling ‘ce_path_match’ from ‘oneway_diff’
    |  510 |                           idx ? idx : tree,
    |      |                           ~~~~~~~~~~~~~~~~~
    |  511 |                           &revs->prune_data, NULL)) {
    |      |                           ~~~~~~~~~~~~~~~~~~~~~~~~
    |
    +--> ‘ce_path_match’: event 9
           |
           |dir.h:535:19:
           |  535 | static inline int ce_path_match(struct index_state *istate,
           |      |                   ^~~~~~~~~~~~~
           |      |                   |
           |      |                   (9) entry to ‘ce_path_match’
           |
         ‘ce_path_match’: event 10
           |
           |  541 |                               S_ISDIR(ce->ce_mode) || S_ISGITLINK(ce->ce_mode));
           |      |                                         ^
           |      |                                         |
           |      |                                         (10) dereference of NULL ‘ce

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 diff-lib.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/diff-lib.c b/diff-lib.c
index ca085a03efc..8373ad7e3ea 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -506,6 +506,9 @@ static int oneway_diff(const struct cache_entry * const *src,
 	if (tree == o->df_conflict_entry)
 		tree = NULL;
 
+	if (!idx && !tree)
+		return 0;
+
 	if (ce_path_match(revs->diffopt.repo->index,
 			  idx ? idx : tree,
 			  &revs->prune_data, NULL)) {
-- 
2.36.1.1124.g577fa9c2ebd