* libc/stdio/snprintf.c (snprintf, _snprintf_r): Fixed code
so size of 0 results in nothing being written to string.
Also fixed code so that when size is non-zero, there is only
a maximum of size - 1 characters written to the array and
a nul terminator is appended at the end.
* libc/stdio/vsnprintf.c (vsnprintf, _vsnprintf_r): Ditto.