filmgrain: clear a field missed in previous fix

BUG=aomedia:2296

Change-Id: Ia40c16f74c7e5c4cecca50e8a21577489d78393e
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 3fae217..78e07da 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -544,6 +544,7 @@
   pars->num_cb_points = 0;
   pars->cb_mult = 0;
   pars->cb_luma_mult = 0;
+  pars->chroma_scaling_from_luma = 0;
   memset(pars->scaling_points_cb, 0, sizeof(pars->scaling_points_cb));
   memset(pars->ar_coeffs_cb, 0, sizeof(pars->ar_coeffs_cb));
 }