Always check parse_tree*()'s return value

Otherwise we may easily run into serious crashes: For example, if we run
`init_tree_desc()` directly after a failed `parse_tree()`, we are
accessing uninitialized data or trying to dereference `NULL`.

Note that the `parse_tree()` function already takes care of showing an
error message. The `parse_tree_indirectly()` and
`repo_get_commit_tree()` functions do not, therefore those latter call
sites need to show a useful error message while the former do not.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge.c

@@ -80,7 +80,10 @@ int checkout_fast_forward(struct repository *r,
 		return -1;
 	}
 	for (i = 0; i < nr_trees; i++) {
-		parse_tree(trees[i]);
+		if (parse_tree(trees[i]) < 0) {
+			rollback_lock_file(&lock_file);
+			return -1;
+		}
 		init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
 	}