fix return value computation in one code path of wcsnrtombs

the affected code was wrongly counting characters instead of bytes.
don't suppress sign output for NANs in printf

formally, it seems a sign is only required when the '+' modifier
appears in the format specifier, in which case either '+' or '-' must
be present in the output. but the specification is written such that
an optional negative sign is part of the output format anyway, and the
simplest approach to fixing the problem is removing the code that was
suppressing the sign.