hb_get_best_mixdown: fix out-of-array reads.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5420 b64f7644-9d1e-0410-96f1-a4d463321fa5
Fix log pollution when Auto Passthru is not possible and uses the fallback audio encoder, if that encoder does not accept s16(p) input samples.

"work: track 1, dithering not supported by codec"



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5421 b64f7644-9d1e-0410-96f1-a4d463321fa5