[PATCH] shallow.c: Don't free unallocated slabs

[Ali Utku Selen <auselen@gmail.com>]

Fix possible segfault when cloning a submodule shallow.

Signed-off-by: Ali Utku Selen <auselen@gmail.com>
---
It is possible to have unallocated slabs in shallow.c's commit_depth
for a shallow submodule with many commits.

Easiest way to reproduce this I found was changing COMMIT_SLAB_SIZE to
32 and run t7406-submodule-update.sh. Segfault happens in case 50:
"submodule update clone shallow submodule outside of depth"

 shallow.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/shallow.c b/shallow.c
index 5fa2b15d37..c33ab94bd7 100644
--- a/shallow.c
+++ b/shallow.c
@@ -156,6 +156,8 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 	for (i = 0; i < depths.slab_count; i++) {
 		int j;
 
+		if (!depths.slab[i])
+			continue;
 		for (j = 0; j < depths.slab_size; j++)
 			free(depths.slab[i][j]);
 	}
-- 
2.17.1