[NORMATIVE] Properly reset mode_deltas

Commit https://aomedia-review.googlesource.com/c/aom/+/52001
introduced a bug where the av1_set_default_mode_deltas() function
does not properly clear the mode_deltas array.

This patch fixes a typo and makes it work properly again.

BUG=aomedia:1632

Change-Id: I697653bd26c743ead22bc042b5e4a63ffc224c70

av1/common/entropymode.c

diff --git a/av1/common/entropymode.c b/av1/common/entropymode.c
index 6da336b..56ee030 100644
--- a/av1/common/entropymode.c
+++ b/av1/common/entropymode.c
@@ -1161,7 +1161,7 @@
   assert(mode_deltas != NULL);
 
   mode_deltas[0] = 0;
-  mode_deltas[0] = 0;
+  mode_deltas[1] = 0;
 }
 
 static void set_default_lf_deltas(struct loopfilter *lf) {