wmaprodec; fix get_bits(0) case.

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

libavcodec/wmaprodec.c

@@ -1130,7 +1130,7 @@ static int decode_subframe(WMAProDecodeCtx *s)
         int num_fill_bits;
         if (!(num_fill_bits = get_bits(&s->gb, 2))) {
             int len = get_bits(&s->gb, 4);
-            num_fill_bits = get_bits(&s->gb, len) + 1;
+            num_fill_bits = (len ? get_bits(&s->gb, len) : 0) + 1;
         }
 
         if (num_fill_bits >= 0) {