engine: java.lang.NullPointerException for SpmStatusVDS

The NullPointerException is occurred because of internal race between SPM stop
and monitoring of storage pool.
The race is not dangereous, so as workaround I solved an exception.
A race will be solved in the future

Change-Id: I4417d1721cfe8365780345e01dbe84c6636cca6e
Bug-Url: https://bugzilla.redhat.com/920736
Signed-off-by: Michael Kublin <mkublin@redhat.com>