ext rc: make ref idx 0 if it's -1

Bug: 461792087
Change-Id: I4060d5be9cb1d4cb8fae59a081c8a643476d599e

av1/encoder/encode_strategy.c

diff --git a/av1/encoder/encode_strategy.c b/av1/encoder/encode_strategy.c
index 22b907c..e821162 100644
--- a/av1/encoder/encode_strategy.c
+++ b/av1/encoder/encode_strategy.c
@@ -1002,6 +1002,11 @@
             (int)cpi->ppi->gf_group.ref_frame_list[gf_index][rf];
       }
     }
+    for (int i = 0; i < REF_FRAMES; ++i) {
+      if (remapped_ref_idx[i] == INVALID_IDX) {
+        remapped_ref_idx[i] = 0;
+      }
+    }
     return;
   }