dnxhdenc: fix signed overflows

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/dnxhdenc.c

@@ -682,7 +682,8 @@ static int dnxhd_encode_rdo(AVCodecContext *avctx, DNXHDEncContext *ctx)
                 int qscale = 1;
                 int mb = y*ctx->m.mb_width+x;
                 for (q = 1; q < avctx->qmax; q++) {
-                    unsigned score = ctx->mb_rc[q][mb].bits*lambda+(ctx->mb_rc[q][mb].ssd<<LAMBDA_FRAC_BITS);
+                    unsigned score = ctx->mb_rc[q][mb].bits*lambda+
+                        ((unsigned)ctx->mb_rc[q][mb].ssd<<LAMBDA_FRAC_BITS);
                     if (score < min) {
                         min = score;
                         qscale = q;
@@ -709,7 +710,7 @@ static int dnxhd_encode_rdo(AVCodecContext *avctx, DNXHDEncContext *ctx)
                 lambda = (lambda+last_higher)>>1;
             else
                 lambda -= down_step;
-            down_step *= 5; // XXX tune ?
+            down_step = FFMIN((int64_t)down_step*5, INT_MAX);
             up_step = 1<<LAMBDA_FRAC_BITS;
             lambda = FFMAX(1, lambda);
             if (lambda == last_lower)