core: fix NPE in VdsUpdateRunTimeInfo

When a disk reported by VDSM doesn't exist in the engine, it causes NPE
when updating the disk statistics. Fixing it to check first if the
returned DB disk is null.

Change-Id: I9a3a02485766253fa63eb3260f60b83899bfd317
Signed-off-by: Oved Ourfali <oourfali@redhat.com>