Write outside buffer in PJLIB's win32 error string

[bennylp]

The PJLIB's Win32 version of platform_strerror() may write data outside the buffer (on buffer[-1] location to be precise) when the function is unable to retrieve the error string using FormatMessage function.

This is caused by the function naively assumes that snprintf will never return negative value:

 len = pj_ansi_snprintf(buffer, bufsize, ...);
 buffer[len] = '\0';

[bennylp]

Fixed in r954.