wmalosslessdec: fix a get_bits(0) in decode_ac_filter

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/wmalosslessdec.c

@@ -407,7 +407,7 @@ static void decode_ac_filter(WmallDecodeCtx *s)
     s->acfilter_scaling = get_bits(&s->gb, 4);
 
     for (i = 0; i < s->acfilter_order; i++)
-        s->acfilter_coeffs[i] = get_bits(&s->gb, s->acfilter_scaling) + 1;
+        s->acfilter_coeffs[i] = (s->acfilter_scaling ? get_bits(&s->gb, s->acfilter_scaling) : 0) + 1;
 }
 
 static void decode_mclms(WmallDecodeCtx *s)