FPMT: Handle the release of previously used cur_frame

Previously used frame buffer of frame_parallel_level 1 frame
is released in av1_init_parallel_frame_context(), before launching
the parallel frame encodes.

Change-Id: I76261e630a1dba0256019f4d635fa7e95207c723

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index e2d6b3d..5a2d315 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -4483,6 +4483,12 @@
 
   assert(*ref_buffers_used_map == 0);
 
+  // Release the previously used frame-buffer by a frame_parallel_level 1 frame.
+  if (first_cpi->common.cur_frame != NULL) {
+    --first_cpi->common.cur_frame->ref_count;
+    first_cpi->common.cur_frame = NULL;
+  }
+
   RefFrameMapPair ref_frame_map_pairs[REF_FRAMES];
   RefFrameMapPair first_ref_frame_map_pairs[REF_FRAMES];
   init_ref_map_pair(first_cpi, first_ref_frame_map_pairs);