fast-import: fix segfault in store_tree()

Branch tree is NULLified by filedelete command if we are trying
to delete root tree. Add sanity check and use load_tree() in that case.

Signed-off-by: Maxim Bublis <satori@yandex-team.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1419,14 +1419,18 @@ static void mktree(struct tree_content *t, int v, struct strbuf *b)
 
 static void store_tree(struct tree_entry *root)
 {
-	struct tree_content *t = root->tree;
+	struct tree_content *t;
 	unsigned int i, j, del;
 	struct last_object lo = { STRBUF_INIT, 0, 0, /* no_swap */ 1 };
 	struct object_entry *le = NULL;
 
 	if (!is_null_sha1(root->versions[1].sha1))
 		return;
 
+	if (!root->tree)
+		load_tree(root);
+	t = root->tree;
+
 	for (i = 0; i < t->entry_count; i++) {
 		if (t->entries[i]->tree)
 			store_tree(t->entries[i]);

t/t9300-fast-import.sh

@@ -3107,7 +3107,7 @@ D ""
 
 INPUT_END
 
-test_expect_failure 'U: filedelete root succeeds' '
+test_expect_success 'U: filedelete root succeeds' '
     git fast-import <input
 '
 
@@ -3117,7 +3117,7 @@ EOF
 
 git diff-tree -M -r U^1 U >actual
 
-test_expect_failure 'U: validate root delete result' '
+test_expect_success 'U: validate root delete result' '
 	compare_diff_raw expect actual
 '