There are two reasons why read_tree_internal might return a NULL
tree. The first one is a corrupt index, but the second one is an
invalidated TREE extension. Up to now, its only way to communicate
with its caller was through the return value being NULL or not.

Allow read_tree_internal to report its exit status independently from
the tree pointer.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>