aaccoder: prevent crash of anmr coder

If minq is negative, the range of sf_idx can be larger than
SCALE_MAX_DIFF allows, causing assertion failures later in
encode_scale_factors.
Reviewed-by: Claudio Freire <klaussfreire@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/aaccoder.c

@@ -370,7 +370,7 @@ static void search_for_quantizers_anmr(AVCodecContext *avctx, AACEncContext *s,
     }
     while (idx) {
         sce->sf_idx[bandaddr[idx]] = minq + q0;
-        minq = paths[idx][minq].prev;
+        minq = FFMAX(paths[idx][minq].prev, 0);
         idx--;
     }
     //set the same quantizers inside window groups