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-recursive.c

@@ -410,7 +410,8 @@ static inline int merge_detect_rename(struct merge_options *opt)
 
 static void init_tree_desc_from_tree(struct tree_desc *desc, struct tree *tree)
 {
-	parse_tree(tree);
+	if (parse_tree(tree) < 0)
+		exit(128);
 	init_tree_desc(desc, tree->buffer, tree->size);
 }