Fix av1_receive_compressed_data error reporting.

The caller of av1_receive_compressed_data() expects it to set
cm->error.error_code to a nonzero error code and return a nonzero value
on error. So av1_receive_compressed_data() should do that after a
get_free_fb() failure.

Document this contract of av1_receive_compressed_data().

BUG=oss-fuzz:9288

Change-Id: I57c8725785182fd9d0993cd3d5cd87f0e0bf004f

av1/decoder/decoder.c

diff --git a/av1/decoder/decoder.c b/av1/decoder/decoder.c
index cad6c29..ef0d5e4 100644
--- a/av1/decoder/decoder.c
+++ b/av1/decoder/decoder.c
@@ -426,7 +426,10 @@
 
   // Find a free frame buffer. Return error if can not find any.
   cm->new_fb_idx = get_free_fb(cm);
-  if (cm->new_fb_idx == INVALID_IDX) return AOM_CODEC_MEM_ERROR;
+  if (cm->new_fb_idx == INVALID_IDX) {
+    cm->error.error_code = AOM_CODEC_MEM_ERROR;
+    return 1;
+  }
 
   // Assign a MV array to the frame buffer.
   cm->cur_frame = &pool->frame_bufs[cm->new_fb_idx];

av1/decoder/decoder.h

diff --git a/av1/decoder/decoder.h b/av1/decoder/decoder.h
index 9efc4f3..55a7f77 100644
--- a/av1/decoder/decoder.h
+++ b/av1/decoder/decoder.h
@@ -206,6 +206,8 @@
   int allocated_row_mt_sync_rows;
 } AV1Decoder;
 
+// Returns 0 on success. Sets pbi->common.error.error_code to a nonzero error
+// code and returns a nonzero value on failure.
 int av1_receive_compressed_data(struct AV1Decoder *pbi, size_t size,
                                 const uint8_t **dest);