r25273: unistrX_to_ascii calls pull_ucs2 - ensure it's never

r25273: unistrX_to_ascii calls pull_ucs2 - ensure it's never
called with -1 (these calls were wrong anyway, target
was an fstring, not a pstring). Found by  Michael Adam <ma@sernet.de>,
now to check all other uses.
Michael - this version uses sizeof(target) not sizeof(fstring).
This way is more future proof.
Jeremy.
(This used to be commit 9ed3046633b6949c68c9aed61b8e9444601cf101)