Call av1_setup_frame_size() when dropping a frame

If av1_rc_drop_frame(cpi) returns true, call av1_setup_frame_size(cpi)
so that the frame buffer cm->cur_frame->buf is allocated.

This CL (for the applejack branch) merges this change from
commit 1629a4b33cab5987af0194de7f964e6f709f4a1b on the master branch:
https://aomedia-review.googlesource.com/c/aom/+/113004

The reason I need this change on the applejack branch is that I am
planning to merge commit 23d914b7ce92dd115a9f5334c9423951b9a6a03c to the
applejack branch and somehow that commit depends on this change.

Change-Id: Ib31f9c66bb48097bf3c8eabbf54dba18cdc4247e
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 6f8774b9..6406afd 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -6371,6 +6371,7 @@
   if (has_no_stats_stage(cpi) && oxcf->rc_mode == AOM_CBR &&
       current_frame->frame_type != KEY_FRAME) {
     if (av1_rc_drop_frame(cpi)) {
+      av1_setup_frame_size(cpi);
       av1_rc_postencode_update_drop_frame(cpi);
       release_scaled_references(cpi);
       return AOM_CODEC_OK;