Adjust mbutils.c so it won't get broken by future pgindent runs.

Adjust mbutils.c so it won't get broken by future pgindent runs.
To do that, replace L'\0' by (WCHAR) 0.  Perhaps someday we should teach
pgindent about wide-character literals, but so long as this is the only
use-case in the entire Postgres sources, a workaround seems easier.