check qblur > 1 (prevent assert failure / segfault)

fixes sf bug #797610

Originally committed as revision 9166 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ratecontrol.c

@@ -201,6 +201,10 @@ int ff_rate_control_init(MpegEncContext *s)
         rcc->pass1_rc_eq_output_sum= 0.001;
         rcc->pass1_wanted_bits=0.001;
 
+        if(s->avctx->qblur > 1.0){
+            av_log(s->avctx, AV_LOG_ERROR, "qblur too large\n");
+            return -1;
+        }
         /* init stuff with the user specified complexity */
         if(s->avctx->rc_initial_cplx){
             for(i=0; i<60*30; i++){