core: avoid NPE in VdsUpdateRunTimeInfo

adding missing NPE check for wrapped primitive value
stat.getusage_mem_percent() (Integer)

Change-Id: I3e32a45ed326de475a38541f5834956148e03054
Bug-Url: https://bugzilla.redhat.com/920986
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>