net: fix kernel_pid_t checks

This was missing after #1374 got merged - otherwise, e.g., 6LoWPAN is not working.

KERNEL_PID_NULL is a negative number. If kernel_pid_t variables are
initialized to this value, one have to check for lt/gte 0.
Seconded. I guess a more fitting name for the macro would have been `KERNEL_PID_UNDEF` or so. The `_NULL` part is somewhat misleading.