Fix assert in MultiMooseEnum (#6769)
Fix assert in MultiMooseEnum (#6769)

jessecarterMOOSE/moose,stimpsonsg/moose,Chuban/moose,jessecarterMOOSE/moose,liuwenf/moose,milljm/moose,sapitts/moose,milljm/moose,SudiptaBiswas/moose,laagesen/moose,idaholab/moose,jasondhales/moose,harterj/moose,permcody/moose,lindsayad/moose,idaholab/moose,dschwen/moose,harterj/moose,stimpsonsg/moose,YaqiWang/moose,laagesen/moose,liuwenf/moose,jasondhales/moose,permcody/moose,joshua-cogliati-inl/moose,andrsd/moose,lindsayad/moose,dschwen/moose,milljm/moose,backmari/moose,nuclear-wizard/moose,jessecarterMOOSE/moose,sapitts/moose,bwspenc/moose,stimpsonsg/moose,SudiptaBiswas/moose,katyhuff/moose,yipenggao/moose,backmari/moose,sapitts/moose,lindsayad/moose,jessecarterMOOSE/moose,YaqiWang/moose,friedmud/moose,liuwenf/moose,jhbradley/moose,milljm/moose,backmari/moose,andrsd/moose,idaholab/moose,jhbradley/moose,yipenggao/moose,katyhuff/moose,laagesen/moose,dschwen/moose,idaholab/moose,yipenggao/moose,idaholab/moose,bwspenc/moose,bwspenc/moose,laagesen/moose,lindsayad/moose,friedmud/moose,joshua-cogliati-inl/moose,liuwenf/moose,bwspenc/moose,stimpsonsg/moose,SudiptaBiswas/moose,friedmud/moose,dschwen/moose,liuwenf/moose,sapitts/moose,jessecarterMOOSE/moose,Chuban/moose,yipenggao/moose,sapitts/moose,backmari/moose,harterj/moose,permcody/moose,nuclear-wizard/moose,permcody/moose,jhbradley/moose,YaqiWang/moose,laagesen/moose,andrsd/moose,andrsd/moose,bwspenc/moose,dschwen/moose,friedmud/moose,jasondhales/moose,jhbradley/moose,YaqiWang/moose,nuclear-wizard/moose,SudiptaBiswas/moose,harterj/moose,Chuban/moose,katyhuff/moose,milljm/moose,joshua-cogliati-inl/moose,liuwenf/moose,joshua-cogliati-inl/moose,andrsd/moose,SudiptaBiswas/moose,katyhuff/moose,nuclear-wizard/moose,jasondhales/moose,lindsayad/moose,harterj/moose,Chuban/moose