decode: fix negative shifts
decode: fix negative shifts

luctrudeau/daala,HeadhunterXamd/daala,ycho/daala,xiph/daala,ycho/daala,jmvalin/daala,HeadhunterXamd/daala,ycho/daala,luctrudeau/daala,HeadhunterXamd/daala,xiphmont/daala,KyleSiefring/daala,jmvalin/daala,xiphmont/daala,jmvalin/daala,luctrudeau/daala,HeadhunterXamd/daala,HeadhunterXamd/daala,KyleSiefring/daala,xiph/daala,xiph/daala,xiphmont/daala,xiph/daala,KyleSiefring/daala,KyleSiefring/daala,xiphmont/daala,luctrudeau/daala,xiph/daala,jmvalin/daala,ycho/daala,luctrudeau/daala,ycho/daala,jmvalin/daala,KyleSiefring/daala,xiphmont/daala