Fix unix_convert() for "*" after changing map_nt_error_from_unix().

Fix unix_convert() for "*" after changing map_nt_error_from_unix().

map_nt_error_from_unix() now assumes that it is called in
an error path and returns an error even for a given errno == 0.
The original behaviour of unix_convert() used the mapping
of errno == 0 ==> NT_STATUS_OK to return success through
an error path.

I think this must have been an oversight, and unix_convert() worked
only by coincidence (or because explicitly using the knowledge
of the conceptually wrong working of map_nt_error_from_unix().

This patch puts this straight by not interpreting errno == 0
as an error condition and proceeding in that case.

Jeremy - please check!

Michael