Squash a prefast warning
Squash a prefast warning

Prefast warns that following the Flink pointer of the EVTCHN context list
may be unsafe due to it being NULL. It should never be NULL so this patch
adds an ASSERTion and hence squashes the warning.

Signed-off-by: Paul Durrant <dba6cd059c0054f8a3c62ba902cd9d7238853ee4@citrix.com>

omeg/xenbus,MarkKMueller/xenbus,MarkKMueller/xenbus,MarkKMueller/xenbus,QubesOS/qubes-vmm-xen-win-pvdrivers-xenbus,omeg/xenbus,omeg/xenbus,rneilturner/xenbus,rneilturner/xenbus,QubesOS/qubes-vmm-xen-win-pvdrivers-xenbus,rneilturner/xenbus,QubesOS/qubes-vmm-xen-win-pvdrivers-xenbus