netifd: fix undefined va_list value which can cause crashes

[mikma]

Reinitialize the va_list value after the call to netifd_udebug_vprintf() in netifd_log_message().

It's needed since netifd_udebug_vprintf() invokes vsnprintf() which in turn invokes the va_arg() macro, and after that call the va_list value is undefined.

[nbd168]

Applied, thanks