avcodec/smacker: Remove redundant check when decoding header trees

When length is zero for a leaf node (which happens iff the Huffman tree
consists of one leaf node only), prefix is also automatically zero.

Performance impact is negligible: For GCC 9 and the sample from #2425,
the time for one call to smka_decode_frame() decreased from 2053758 to
2053671 decicycles; for Clang 9 it went from 1523153 to 1521288.

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

libavcodec/smacker.c

@@ -105,13 +105,8 @@ static int smacker_decode_tree(GetBitContext *gb, HuffContext *hc, uint32_t pref
             av_log(NULL, AV_LOG_ERROR, "Tree size exceeded!\n");
             return AVERROR_INVALIDDATA;
         }
-        if(length){
         hc->bits[hc->current]    = prefix;
         hc->lengths[hc->current] = length;
-        } else {
-            hc->bits[hc->current] = 0;
-            hc->lengths[hc->current] = 0;
-        }
         hc->values[hc->current] = get_bits(gb, 8);
         hc->current++;
         return 0;