Handle kernel_pid_t consistently

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.
Compare https://github.com/RIOT-OS/RIOT/pull/1549#discussion_r15933253