Fix uid_to_sid mapping when the idmap cache is empty.

Fix uid_to_sid mapping when the idmap cache is empty.

This failed for backends other than passed, since
idmap_backends_unixid_to_sid() always asked passdb first,
which returned Success no matter whether a mapping was
found or not.

One effect wast that getpwuid failed after "net cache flush".
Only after filling the cache with a getpwnam call it succeeded.

This fix makes the behaviour of idmap_backends_unixid_to_sid()
exactly the same as that of idmap_backends_sid_to_unixid()

Michael
(cherry picked from commit da7eb5e22e56ea718e393375085cf66a90afafc8)

Signed-off-by: Michael Adam <obnox@samba.org>