Prevent crash trying to freeze script task with no page.
Prevent crash trying to freeze script task with no page.

This fixes a crash resulting from a race between loading an initial
document and navigating to a subsequent document. If the navigation
happens before the initial document has had a chance to create its
root page, we crash trying to unwrap a None. Note that the are likely
further similar timing issues with more complex sequences of navigation
and history manipulation.

upsuper/servo,upsuper/servo,KiChjang/servo,rnestler/servo,nnethercote/servo,paulrouget/servo,jimberlage/servo,CJ8664/servo,DominoTree/servo,saneyuki/servo,mbrubeck/servo,pyfisch/servo,nnethercote/servo,emilio/servo,pyfisch/servo,peterjoel/servo,KiChjang/servo,RenaudParis/servo,DominoTree/servo,canaltinova/servo,eddyb/servo,SimonSapin/servo,sadmansk/servo,paulrouget/servo,mattnenterprise/servo,RenaudParis/servo,runarberg/servo,notriddle/servo,thiagopnts/servo,eddyb/servo,j3parker/servo,SimonSapin/servo,paulrouget/servo,upsuper/servo,splav/servo,dsandeephegde/servo,splav/servo,RenaudParis/servo,paulrouget/servo,RenaudParis/servo,runarberg/servo,dati91/servo,larsbergstrom/servo,dsandeephegde/servo,cbrewster/servo,steveklabnik/servo,splav/servo,rnestler/servo,DominoTree/servo,ConnorGBrewster/servo,saneyuki/servo,pyfisch/servo,larsbergstrom/servo,steveklabnik/servo,paulrouget/servo,pyfisch/servo,peterjoel/servo,avadacatavra/servo,cbrewster/servo,nrc/servo,mattnenterprise/servo,runarberg/servo,saneyuki/servo,canaltinova/servo,runarberg/servo,anthgur/servo,dsandeephegde/servo,anthgur/servo,avadacatavra/servo,anthgur/servo,steveklabnik/servo,anthgur/servo,peterjoel/servo,SimonSapin/servo,szeged/servo,fiji-flo/servo,larsbergstrom/servo,notriddle/servo,rnestler/servo,upsuper/servo,j3parker/servo,notriddle/servo,larsbergstrom/servo,KiChjang/servo,nrc/servo,dsandeephegde/servo,mbrubeck/servo,notriddle/servo,jimberlage/servo,nnethercote/servo,canaltinova/servo,pyfisch/servo,eddyb/servo,runarberg/servo,CJ8664/servo,avadacatavra/servo,SimonSapin/servo,jimberlage/servo,upsuper/servo,mbrubeck/servo,emilio/servo,nnethercote/servo,anthgur/servo,emilio/servo,rnestler/servo,larsbergstrom/servo,mbrubeck/servo,KiChjang/servo,larsbergstrom/servo,runarberg/servo,szeged/servo,avadacatavra/servo,pyfisch/servo,emilio/servo,pyfisch/servo,paulrouget/servo,j3parker/servo,cbrewster/servo,szeged/servo,ConnorGBrewster/servo,upsuper/servo,rnestler/servo,SimonSapin/servo,upsuper/servo,nrc/servo,szeged/servo,cbrewster/servo,notriddle/servo,DominoTree/servo,mattnenterprise/servo,peterjoel/servo,jimberlage/servo,saneyuki/servo,upsuper/servo,dati91/servo,peterjoel/servo,pyfisch/servo,sadmansk/servo,dsandeephegde/servo,DominoTree/servo,anthgur/servo,sadmansk/servo,thiagopnts/servo,rnestler/servo,CJ8664/servo,canaltinova/servo,splav/servo,KiChjang/servo,saneyuki/servo,RenaudParis/servo,larsbergstrom/servo,pyfisch/servo,szeged/servo,jimberlage/servo,CJ8664/servo,anthgur/servo,ConnorGBrewster/servo,thiagopnts/servo,fiji-flo/servo,nrc/servo,peterjoel/servo,ConnorGBrewster/servo,sadmansk/servo,emilio/servo,thiagopnts/servo,larsbergstrom/servo,emilio/servo,ConnorGBrewster/servo,notriddle/servo,ConnorGBrewster/servo,avadacatavra/servo,nnethercote/servo,dsandeephegde/servo,szeged/servo,fiji-flo/servo,CJ8664/servo,saneyuki/servo,splav/servo,splav/servo,canaltinova/servo,j3parker/servo,eddyb/servo,steveklabnik/servo,paulrouget/servo,emilio/servo,szeged/servo,saneyuki/servo,peterjoel/servo,avadacatavra/servo,mattnenterprise/servo,DominoTree/servo,nnethercote/servo,peterjoel/servo,steveklabnik/servo,KiChjang/servo,nrc/servo,CJ8664/servo,eddyb/servo,RenaudParis/servo,paulrouget/servo,dati91/servo,emilio/servo,eddyb/servo,notriddle/servo,sadmansk/servo,RenaudParis/servo,avadacatavra/servo,KiChjang/servo,fiji-flo/servo,notriddle/servo,nnethercote/servo,mbrubeck/servo,splav/servo,cbrewster/servo,mattnenterprise/servo,fiji-flo/servo,nnethercote/servo,thiagopnts/servo,fiji-flo/servo,DominoTree/servo,j3parker/servo,anthgur/servo,pyfisch/servo,dati91/servo,canaltinova/servo,jimberlage/servo,splav/servo,notriddle/servo,eddyb/servo,jimberlage/servo,paulrouget/servo,splav/servo,dati91/servo,steveklabnik/servo,dsandeephegde/servo,KiChjang/servo,larsbergstrom/servo,DominoTree/servo,thiagopnts/servo,splav/servo,mattnenterprise/servo,fiji-flo/servo,KiChjang/servo,peterjoel/servo,dati91/servo,rnestler/servo,j3parker/servo,mbrubeck/servo,saneyuki/servo,dati91/servo,szeged/servo,j3parker/servo,peterjoel/servo,emilio/servo,thiagopnts/servo,szeged/servo,cbrewster/servo,dsandeephegde/servo,fiji-flo/servo,thiagopnts/servo,larsbergstrom/servo,sadmansk/servo,saneyuki/servo,cbrewster/servo,SimonSapin/servo,DominoTree/servo,jimberlage/servo,mbrubeck/servo,CJ8664/servo,nrc/servo,canaltinova/servo,SimonSapin/servo,cbrewster/servo,DominoTree/servo,saneyuki/servo,nnethercote/servo,paulrouget/servo,notriddle/servo,nrc/servo,avadacatavra/servo,steveklabnik/servo,ConnorGBrewster/servo,CJ8664/servo,ConnorGBrewster/servo,eddyb/servo,SimonSapin/servo,szeged/servo,jimberlage/servo,mattnenterprise/servo,mattnenterprise/servo,KiChjang/servo,sadmansk/servo,emilio/servo,mbrubeck/servo,sadmansk/servo,canaltinova/servo,runarberg/servo,nnethercote/servo,dati91/servo,rnestler/servo