Fix for a premature deletion bug. The _arg SGPropertyNode* is passed

Fix for a premature deletion bug.  The _arg SGPropertyNode* is passed
to handlers which might want to assign it to a SGPropertyNode_ptr for
reference counting (Nasal does, for instance, to prevent garbage
collector interactions).  If that smart pointer is then destroyed,
that will free this object while it is still live.

Simply use a SGPropertyNode_ptr here; the code ends up smaller as a
bonus, since FGBinding no longer has to deallocation for _arg.