Two tweaks to fix wide string copy errors exposed by dlmalloc.

Handling of the distinguished value for an empty c_string_copy was not fully implemented
in the remote string routines.  Wide strings should report a length of zero if the string
itself is a NULL.  If the remote string pointer is null, the local one should be, too.
Merge pull request #633 from hildeth/master

Two tweaks to fix wide string copy errors exposed by dlmalloc.

Handling of the distinguished value for an empty c_string_copy was not fully implemented
in the remote string routines. Wide strings should report a length of zero if the string
itself is a NULL. If the remote string pointer is null, the local one should be, too.