Fixed format string misuse (V618) and not zero-terminated string (C6053) warnings in win32 'warnx' implementation

Both warnings are false positive, since `opterrmsg` buffer is controled solely by the `warnx` function and `_vsnprintf` appears to be zero-terminating its output as the documentation says. I've changed the `fprintf` call so it guards from a formatting string input to make V618 of the PVS Studio happy, manually zeroed the buffer before `_vsnprintf` call and limited it's output size to (buffer size - 1) to guarantee the output is always zero-terminated despite the implementation and suppressed the warning C6053 of the VS Code Analysis.