avformat/av1dec: fix return value on some code paths

If avio_read() returns a value of bytes read that's lower than the
expected, return an error instead. And when there are zero bytes in
the prefetch buffer, return 0 in order for the frame merge bsf to
drain all potentially buffered packets.

Missed by mistake when amending and committing 9a7bdb6d71.

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/av1dec.c

@@ -411,7 +411,7 @@ static int obu_read_data(AVFormatContext *s, AVPacket *pkt, int len)
         ret = avio_read(s->pb, pkt->data + size, left);
         if (ret != left) {
             av_log(c, AV_LOG_ERROR, "Failed to read %d frome file\n", left);
-            return ret;
+            return ret < 0 ? ret : AVERROR_INVALIDDATA;
         }
     }
     return 0;
@@ -426,7 +426,7 @@ static int obu_get_packet(AVFormatContext *s, AVPacket *pkt)
 
     ret = obu_prefetch(s, header);
     if (!ret)
-        return AVERROR(EOF);
+        return 0;
 
     ret = read_obu_with_size(header, ret, &obu_size, &type);
     if (ret < 0) {