Avoid a null pointer dereference in avcodec_decode_video2().

This could happen if oom occured while probing a file.
Reported in ticket #1900.

libavcodec/utils.c

@@ -1920,6 +1920,8 @@ int attribute_align_arg avcodec_decode_video2(AVCodecContext *avctx, AVFrame *pi
     // copy to ensure we do not change avpkt
     AVPacket tmp = *avpkt;
 
+    if (!avctx->codec)
+        return AVERROR(EINVAL);
     if (avctx->codec->type != AVMEDIA_TYPE_VIDEO) {
         av_log(avctx, AV_LOG_ERROR, "Invalid media type for video\n");
         return AVERROR(EINVAL);