smbd: Fix previous_slash()

smbd: Fix previous_slash()

Untested code is broken code... previous_slash() did not return a
pointer to the slash but after it. This went undetected because so far
we never call symlink_target_path() with "unparsed==0". Once we
started doing that, we would find that the "unparsed==0" case actually
puts parent on the "previous slash", not the character behind it.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>