engine: added safety checks when getting Fixture channels by head
engine: added safety checks when getting Fixture channels by head

When creating a GroupHead, if aHead is not specified, then the head
index is equal to -1.
Prototype: GroupHead(quint32 aFxi = Fixture::invalidId(), int aHead = -1);
This might cause disasters around.

nedmech/qlcplus,kripton/qlcplus,mcallegari/qlcplus,sbenejam/qlcplus,plugz/qlcplus,joepadmiraal/qlcplus,mcallegari/qlcplus,kripton/qlcplus,joepadmiraal/qlcplus,nedmech/qlcplus,nedmech/qlcplus,kripton/qlcplus,joepadmiraal/qlcplus,kripton/qlcplus,mcallegari/qlcplus,sbenejam/qlcplus,nedmech/qlcplus,kripton/qlcplus,plugz/qlcplus,plugz/qlcplus,mcallegari/qlcplus,joepadmiraal/qlcplus,sbenejam/qlcplus,sbenejam/qlcplus,joepadmiraal/qlcplus,sbenejam/qlcplus,plugz/qlcplus,nedmech/qlcplus,sbenejam/qlcplus,kripton/qlcplus,mcallegari/qlcplus,nedmech/qlcplus,plugz/qlcplus,mcallegari/qlcplus,sbenejam/qlcplus,joepadmiraal/qlcplus,mcallegari/qlcplus,plugz/qlcplus,plugz/qlcplus,kripton/qlcplus