ratecontrol: replace VLAs with malloc/free

(cherry picked from commit 2c6bd7d1f992989d1cc108534e0896771b86824f)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ratecontrol.c

@@ -537,8 +537,8 @@ static void adaptive_quantization(MpegEncContext *s, double q){
     const float border_masking = s->avctx->border_masking;
     float bits_sum= 0.0;
     float cplx_sum= 0.0;
-    float cplx_tab[s->mb_num];
-    float bits_tab[s->mb_num];
+    float *cplx_tab = av_malloc(s->mb_num * sizeof(*cplx_tab));
+    float *bits_tab = av_malloc(s->mb_num * sizeof(*bits_tab));
     const int qmin= s->avctx->mb_lmin;
     const int qmax= s->avctx->mb_lmax;
     Picture * const pic= &s->current_picture;
@@ -639,6 +639,9 @@ static void adaptive_quantization(MpegEncContext *s, double q){
 //printf("%2d%3d ", intq, ff_sqrt(s->mc_mb_var[i]));
         s->lambda_table[mb_xy]= intq;
     }
+
+    av_free(cplx_tab);
+    av_free(bits_tab);
 }
 
 void ff_get_2pass_fcode(MpegEncContext *s){