Both tree_ascend() and tree_pop() may cause errno to be
set/clobbered. As for the latter, we're certainly not interested in
the random values that free() etc. may leave in errno.

If tree_ascend() fails, its return value takes precedence in
determining t->visit_type and the return value, but tree_ascend() also
updates t->tree_errno appropriately, which makes sense.

But when tree_ascend() succeeds, t->tree_errno should reflect the
reason for opendir() failing, which requires that we record errno
immediately.

Bug bounty:	$20

The relevant change in tar/tree.c was made in:
    2011-08-25 Tarsnap 1.0.30
    c80c7d2

Reported by:	Rasmus Villemoes
Bug bounty:	$11

This should have been part of:
    2008-12-10 Tarsnap 1.0.17
    b8dcf6c

There's lots of code between `tree_ret = tree_next()` and this part,
so we can't trust errno.

Reported by:	Rasmus Villemoes
Bug bounty:	$10