Be more careful calling wcstombs

The function is not guaranteed to NUL-terminate its output.  It
*is*, however, guaranteed not to generate more than two bytes per
multibyte character (plus terminating nul), so the general approach
I'm taking is to try to allocate enough space, AND to manually add a
NUL at the end of each buffer just in case I screwed up the "enough
space" thing.

Fixes bug 5909.