wmaprodec: check num_sfb for validity. Fix out of array accesses

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

libavcodec/wmaprodec.c

@@ -412,6 +412,10 @@ static av_cold int decode_init(AVCodecContext *avctx)
         }
         s->sfb_offsets[i][band - 1] = subframe_len;
         s->num_sfb[i]               = band - 1;
+        if (s->num_sfb[i] <= 0) {
+            av_log(avctx, AV_LOG_ERROR, "num_sfb invalid\n");
+            return AVERROR_INVALIDDATA;
+        }
     }