Avoid null poiter exception in isFreeVRRPGroup.

MP-vcpes don't have VRRP configuration (it's set to null), so
calling isFreeVRRPGroup when there is a MP-vcpe already created
launches a nullpointer exception.

This patch fixes this by checking vrrp configuration and group is
not null before trying to compare it.
Merge pull request #545 from isartcanyameres/fix/null-in-isfree-checker

Avoid null poiter exception in isFreeVRRPGroup.