Fix an interpolation filter assignment

Change-Id: I052a4a667f199b96861f70dc7f9df240561abab7

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 211025a..50cd849 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -11573,7 +11573,7 @@
     rate2 += av1_get_switchable_rate(cpi, xd);
   } else {
 #if CONFIG_DUAL_FILTER
-    for (i = 0; i < 4; ++i) mbmi->interp_filter[0] = cm->interp_filter;
+    for (i = 0; i < 4; ++i) mbmi->interp_filter[i] = cm->interp_filter;
 #else
     mbmi->interp_filter = cm->interp_filter;
 #endif  // CONFIG_DUAL_FILTER