JNI: Add a check any time Device.Signal gets its mapper_signal pointer.
JNI: Add a check any time Device.Signal gets its mapper_signal pointer.

malloch/libmapper,libmapper/libmapper,johnty/libmapper,johnty/libmapper,radarsat1/libmapper,radarsat1/libmapper,libmapper/libmapper,radarsat1/libmapper,libmapper/libmapper,malloch/libmapper,davidhernon/libmapper,malloch/libmapper,radarsat1/libmapper,libmapper/libmapper,davidhernon/libmapper-admin2,malloch/libmapper,davidhernon/libmapper,davidhernon/libmapper-admin2,johnty/libmapper,davidhernon/libmapper,johnty/libmapper,libmapper/libmapper,davidhernon/libmapper,davidhernon/libmapper-admin2,johnty/libmapper,davidhernon/libmapper,libmapper/libmapper,davidhernon/libmapper-admin2,davidhernon/libmapper-admin2,malloch/libmapper,malloch/libmapper,radarsat1/libmapper