Safer Elem::ancestor()
Safer Elem::ancestor()

This should avoid potential errors in ParallelMesh corner cases, and
add consistency checking in DEBUG mode.

libMesh/libmesh,pbauman/libmesh,friedmud/libmesh,cahaynes/libmesh,dknez/libmesh,jiangwen84/libmesh,vikramvgarg/libmesh,coreymbryant/libmesh,svallaghe/libmesh,dmcdougall/libmesh,roystgnr/libmesh,jiangwen84/libmesh,libMesh/libmesh,dknez/libmesh,cahaynes/libmesh,libMesh/libmesh,BalticPinguin/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,vikramvgarg/libmesh,hrittich/libmesh,jwpeterson/libmesh,cahaynes/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,pbauman/libmesh,dschwen/libmesh,dschwen/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,hrittich/libmesh,svallaghe/libmesh,giorgiobornia/libmesh,dknez/libmesh,pbauman/libmesh,permcody/libmesh,hrittich/libmesh,cahaynes/libmesh,hrittich/libmesh,jiangwen84/libmesh,giorgiobornia/libmesh,90jrong/libmesh,90jrong/libmesh,capitalaslash/libmesh,hrittich/libmesh,aeslaughter/libmesh,balborian/libmesh,libMesh/libmesh,pbauman/libmesh,aeslaughter/libmesh,roystgnr/libmesh,vikramvgarg/libmesh,hrittich/libmesh,roystgnr/libmesh,balborian/libmesh,90jrong/libmesh,benkirk/libmesh,giorgiobornia/libmesh,roystgnr/libmesh,balborian/libmesh,capitalaslash/libmesh,giorgiobornia/libmesh,90jrong/libmesh,friedmud/libmesh,benkirk/libmesh,aeslaughter/libmesh,aeslaughter/libmesh,benkirk/libmesh,dschwen/libmesh,coreymbryant/libmesh,coreymbryant/libmesh,libMesh/libmesh,svallaghe/libmesh,balborian/libmesh,capitalaslash/libmesh,benkirk/libmesh,90jrong/libmesh,benkirk/libmesh,capitalaslash/libmesh,jiangwen84/libmesh,aeslaughter/libmesh,dmcdougall/libmesh,dmcdougall/libmesh,vikramvgarg/libmesh,permcody/libmesh,dknez/libmesh,jiangwen84/libmesh,permcody/libmesh,friedmud/libmesh,permcody/libmesh,dmcdougall/libmesh,BalticPinguin/libmesh,friedmud/libmesh,benkirk/libmesh,friedmud/libmesh,libMesh/libmesh,dknez/libmesh,pbauman/libmesh,hrittich/libmesh,90jrong/libmesh,vikramvgarg/libmesh,friedmud/libmesh,svallaghe/libmesh,svallaghe/libmesh,coreymbryant/libmesh,dschwen/libmesh,svallaghe/libmesh,friedmud/libmesh,jwpeterson/libmesh,aeslaughter/libmesh,capitalaslash/libmesh,hrittich/libmesh,cahaynes/libmesh,jwpeterson/libmesh,coreymbryant/libmesh,friedmud/libmesh,balborian/libmesh,permcody/libmesh,balborian/libmesh,BalticPinguin/libmesh,vikramvgarg/libmesh,pbauman/libmesh,friedmud/libmesh,roystgnr/libmesh,dknez/libmesh,dknez/libmesh,aeslaughter/libmesh,dschwen/libmesh,benkirk/libmesh,permcody/libmesh,dmcdougall/libmesh,permcody/libmesh,capitalaslash/libmesh,90jrong/libmesh,cahaynes/libmesh,cahaynes/libmesh,vikramvgarg/libmesh,aeslaughter/libmesh,coreymbryant/libmesh,BalticPinguin/libmesh,libMesh/libmesh,dschwen/libmesh,coreymbryant/libmesh,giorgiobornia/libmesh,90jrong/libmesh,balborian/libmesh,jwpeterson/libmesh,aeslaughter/libmesh,roystgnr/libmesh,dschwen/libmesh,roystgnr/libmesh,dmcdougall/libmesh,jiangwen84/libmesh,pbauman/libmesh,dschwen/libmesh,libMesh/libmesh,dmcdougall/libmesh,jwpeterson/libmesh,cahaynes/libmesh,giorgiobornia/libmesh,vikramvgarg/libmesh,jiangwen84/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,jiangwen84/libmesh,pbauman/libmesh,balborian/libmesh,svallaghe/libmesh,dknez/libmesh,permcody/libmesh,giorgiobornia/libmesh,giorgiobornia/libmesh,dmcdougall/libmesh,pbauman/libmesh,BalticPinguin/libmesh,vikramvgarg/libmesh,roystgnr/libmesh,90jrong/libmesh,benkirk/libmesh,benkirk/libmesh,balborian/libmesh,hrittich/libmesh,capitalaslash/libmesh,svallaghe/libmesh,balborian/libmesh,svallaghe/libmesh,coreymbryant/libmesh