Initialise ext buffer after decoding keyframes

Prior to this patch, buffers for all reference frames except for the
ext buffer were initialised after decoding a keyframe.

This patch includes the ext buffer in this initialisation.

Change-Id: Ia3e1fe38553ea57ba9b279b75b9dd45afd6ab42d
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 535514f..7cef2f2 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -3582,6 +3582,8 @@
                cm->new_fb_idx);
     ref_cnt_fb(pool->frame_bufs, &cm->ref_frame_map[cpi->alt_fb_idx],
                cm->new_fb_idx);
+    ref_cnt_fb(pool->frame_bufs, &cm->ref_frame_map[cpi->ext_fb_idx],
+               cm->new_fb_idx);
   } else if (av1_preserve_existing_gf(cpi)) {
     // We have decided to preserve the previously existing golden frame as our
     // new ARF frame. However, in the short term in function