nls: utf8_wcstombs: use correct buffer size in error case

When utf8_wcstombs encounters a character that cannot be encoded, we
must not decrease the remaining output buffer size because nothing has
been written to the output buffer.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>