switch/nrt: correct some logic if NULL pointer

The test for NRT_NULL_MAGIC failed to capture some problems
if the pointer to the structure was NULL. This is an ammendment
to commit 2a55aa0b1bf5d9a2ff3cd350eae69c29122407bf
Merge branch 'slurm-2.6'

Conflicts:
	src/plugins/switch/nrt/nrt.c