Obey frame_flags for forced key frame

It looks like this was an existing issue for two pass and the
one pass path merge uncovered it. This avoids early termination
on an ALTREF in av1_get_second_pass_params when a keyframe is forced.

BUG=aomedia:2416

Change-Id: Ifd3cfa31d8e3c001ff54a028a05ab3ec28f0021c
diff --git a/av1/encoder/pass2_strategy.c b/av1/encoder/pass2_strategy.c
index 32ad9a5..b49fb04 100644
--- a/av1/encoder/pass2_strategy.c
+++ b/av1/encoder/pass2_strategy.c
@@ -1856,7 +1856,7 @@
 
   if (cpi->oxcf.pass == 2 && !twopass->stats_in) return;
 
-  if (rc->frames_till_gf_update_due > 0) {
+  if (rc->frames_till_gf_update_due > 0 && !(frame_flags & FRAMEFLAGS_KEY)) {
     assert(gf_group->index < gf_group->size);
     const int update_type = gf_group->update_type[gf_group->index];