Clamp rate error estimate.

Add back clamp which ensures that the Q adaptation
is turned off when the over_shoot_pct and under_shoot_pct
parameters are set to 100.

Change-Id: Id0161b114d39a3029cd3eb28020caab0c3914922

vp9/encoder/vp9_firstpass.c

@@ -2478,6 +2478,7 @@ void vp9_twopass_postencode_update(VP9_COMP *cpi) {
   if (rc->total_actual_bits) {
     rc->rate_error_estimate =
       (int)((rc->vbr_bits_off_target * 100) / rc->total_actual_bits);
+    rc->rate_error_estimate = clamp(rc->rate_error_estimate, -100, 100);
   } else {
     rc->rate_error_estimate = 0;
   }