Add more assertions to level 3 MV encoding
Add more assertions to level 3 MV encoding

This should make it harder to introduce desyncs like the one fixed by
by the previous commit.

jmvalin/daala,felipebetancur/daala,KyleSiefring/daala,felipebetancur/daala,ascent12/daala,iankronquist/daala,mbebenita/daala,xiphmont/daala,iankronquist/daala,kodabb/daala,kustom666/daala,jmvalin/daala,tribouille/daala,tribouille/daala,vr000m/daala,xiph/daala,ycho/daala,HeadhunterXamd/daala,kustom666/daala,iankronquist/daala,KyleSiefring/daala,vr000m/daala,nvoron23/daala,jmvalin/daala,tribouille/daala,kodabb/daala,kbara/daala,mbebenita/daala,xiph/daala,mbebenita/daala,HeadhunterXamd/daala,felipebetancur/daala,nvoron23/daala,xiphmont/daala,nvoron23/daala,xiph/daala,luctrudeau/daala,vr000m/daala,iankronquist/daala,kustom666/daala,kbara/daala,kodabb/daala,jmvalin/daala,kustom666/daala,luctrudeau/daala,kodabb/daala,xiph/daala,mbebenita/daala,felipebetancur/daala,KyleSiefring/daala,ascent12/daala,ycho/daala,kbara/daala,ycho/daala,KyleSiefring/daala,nvoron23/daala,luctrudeau/daala,vr000m/daala,nvoron23/daala,ascent12/daala,ycho/daala,ascent12/daala,tribouille/daala,kbara/daala,ascent12/daala,jmvalin/daala,tribouille/daala,kustom666/daala,ycho/daala,xiphmont/daala,iankronquist/daala,kodabb/daala,felipebetancur/daala,xiph/daala,vr000m/daala,luctrudeau/daala,HeadhunterXamd/daala,HeadhunterXamd/daala,HeadhunterXamd/daala,xiphmont/daala,mbebenita/daala,KyleSiefring/daala,luctrudeau/daala,xiphmont/daala