rd_average: error out if output file already exists
rd_average: error out if output file already exists

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