core: fix possible NPE on run/migrate commands

In some cases we might reach the runningFailed method when no host was
selected by the scheduler, so we could get NPE when trying to decrease
the pending memory. So null-check is added.

Change-Id: I664b2aaabc855ed3b28a9b8fdee64d32c4191e65
Signed-off-by: Arik Hadas <ahadas@redhat.com>