RIOT-OS/RIOT — pull request #1549 by OlegHahm (2014-08-05 21:57:44+00:00)
Title: 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.