core: avoid NPE in RecoveryStoragePool command

getStoragePool() might return null which will cause to NPE, this patch
set this check to occur before accessing the storage pool properties.

Change-Id: Ie82c2050989c3ec34ad5ff9f23f853e1894c7ecd
Signed-off-by: Liron Aravot <laravot@redhat.com>