avcodec/smacker: Don't warn for Huffmann tables with one element

avcodec/smacker: Don't warn for Huffmann tables with one element

The Huffmann tables used by Smacker can consist of exactly one leaf only
in which case the length of the corresponding code is zero; there is
then exactly one value encoded. Our VLC can't handle this and therefore
this case needs to be treated separately; it has been implemented in
commit 48cbdaea157671d456750e00fde37c6d7595fad6. Yet said commit also
made the decoder emit an error message (despite not erroring out) in this
case, although it seems that this is rather a limitation of our VLC API.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>