NO-JIRA: Fix ignored python exceptions "Selectable' object has no attribute '_impl"
NO-JIRA: Fix ignored python exceptions "Selectable' object has no attribute '_impl"

Fix errors introduced by:
  e769ad5 NO-JIRA: Fix core dumps and memory management errors in  python binding, engine.py.
Deleted attribute _impl was later being checked for None.

jeckersb/Proton,datawire/qpid-proton,alanconway/qpid-proton,jeckersb/Proton,wprice/qpid-proton,Karm/qpid-proton,clemensv/qpid-proton,prestona/qpid-proton,RobertoMalatesta/qpid-proton,Azure/qpid-proton,Karm/qpid-proton,RobertoMalatesta/qpid-proton,FlaPer87/qpid-proton,jeckersb/Proton,kgiusti/qpid-proton,astitcher/qpid-proton,Azure/qpid-proton,FlaPer87/qpid-proton,wprice/qpid-proton,clemensv/qpid-proton,dcristoloveanu/qpid-proton,astitcher/qpid-proton,dcristoloveanu/qpid-proton,datawire/qpid-proton,clemensv/qpid-proton,wprice/qpid-proton,apache/qpid-proton,apache/qpid-proton,datawire/qpid-proton,ssorj/qpid-proton,prestona/qpid-proton,FlaPer87/qpid-proton,FlaPer87/qpid-proton,prestona/qpid-proton,clemensv/qpid-proton,bozzzzo/qpid-proton,datawire/qpid-proton,Karm/qpid-proton,bozzzzo/qpid-proton,FlaPer87/qpid-proton,jeckersb/Proton,RobertoMalatesta/qpid-proton,RobertoMalatesta/qpid-proton,gemmellr/qpid-proton,RobertoMalatesta/qpid-proton,gemmellr/qpid-proton,bozzzzo/qpid-proton,datawire/qpid-proton,dcristoloveanu/qpid-proton,Karm/qpid-proton,wprice/qpid-proton,prestona/qpid-proton,Karm/qpid-proton,datawire/qpid-proton,jeckersb/Proton,Azure/qpid-proton,Karm/qpid-proton,clemensv/qpid-proton,wprice/qpid-proton,RobertoMalatesta/qpid-proton,clemensv/qpid-proton,Karm/qpid-proton,ssorj/qpid-proton,ChugR/qpid-proton,jeckersb/Proton,gemmellr/qpid-proton-j,Karm/qpid-proton,Karm/qpid-proton,ChugR/qpid-proton,wprice/qpid-proton,bozzzzo/qpid-proton,datawire/qpid-proton,gemmellr/qpid-proton,Azure/qpid-proton,wprice/qpid-proton,prestona/qpid-proton,prestona/qpid-proton,bozzzzo/qpid-proton,ssorj/qpid-proton,bozzzzo/qpid-proton,jeckersb/Proton,dcristoloveanu/qpid-proton,clemensv/qpid-proton,apache/qpid-proton,jeckersb/Proton,ChugR/qpid-proton,FlaPer87/qpid-proton,ChugR/qpid-proton,RobertoMalatesta/qpid-proton,bozzzzo/qpid-proton,dcristoloveanu/qpid-proton,dcristoloveanu/qpid-proton,ChugR/qpid-proton,Karm/qpid-proton,RobertoMalatesta/qpid-proton,wprice/qpid-proton,Azure/qpid-proton,clemensv/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton,jeckersb/Proton,astitcher/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton,astitcher/qpid-proton,alanconway/qpid-proton,alanconway/qpid-proton,prestona/qpid-proton,astitcher/qpid-proton,jeckersb/Proton,prestona/qpid-proton,FlaPer87/qpid-proton,clemensv/qpid-proton,kgiusti/qpid-proton,clemensv/qpid-proton,prestona/qpid-proton,RobertoMalatesta/qpid-proton,RobertoMalatesta/qpid-proton,RobertoMalatesta/qpid-proton,FlaPer87/qpid-proton,Karm/qpid-proton,astitcher/qpid-proton,Azure/qpid-proton,FlaPer87/qpid-proton,wprice/qpid-proton,kgiusti/qpid-proton,Azure/qpid-proton,Azure/qpid-proton,Azure/qpid-proton,dcristoloveanu/qpid-proton,dcristoloveanu/qpid-proton,datawire/qpid-proton,datawire/qpid-proton,dcristoloveanu/qpid-proton,gemmellr/qpid-proton-j,alanconway/qpid-proton,jeckersb/Proton,bozzzzo/qpid-proton,bozzzzo/qpid-proton,jeckersb/Proton,wprice/qpid-proton,dcristoloveanu/qpid-proton,ssorj/qpid-proton,clemensv/qpid-proton,kgiusti/qpid-proton,FlaPer87/qpid-proton,Azure/qpid-proton,alanconway/qpid-proton,kgiusti/qpid-proton,Azure/qpid-proton,datawire/qpid-proton,prestona/qpid-proton,bozzzzo/qpid-proton,FlaPer87/qpid-proton,datawire/qpid-proton,prestona/qpid-proton,kgiusti/qpid-proton,bozzzzo/qpid-proton,alanconway/qpid-proton,Karm/qpid-proton,dcristoloveanu/qpid-proton,dcristoloveanu/qpid-proton,FlaPer87/qpid-proton,prestona/qpid-proton,apache/qpid-proton,datawire/qpid-proton,ChugR/qpid-proton,gemmellr/qpid-proton,wprice/qpid-proton,apache/qpid-proton