checkout, clone: die if tree cannot be parsed

checkout, clone: die if tree cannot be parsed

When a tree oid is invalid, parse_tree_indirect() can return NULL. Check
for NULL instead of proceeding as though it were a valid pointer and
segfaulting.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>