Check that palctrl is available on init instead of crashing when trying to use

it on decode.
Fixes mpeg2/smclockmpeg2.avi.2.1 from issue 1240.

Originally committed as revision 19347 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/qpeg.c

@@ -289,6 +289,10 @@ static int decode_frame(AVCodecContext *avctx,
 static av_cold int decode_init(AVCodecContext *avctx){
     QpegContext * const a = avctx->priv_data;
 
+    if (!avctx->palctrl) {
+        av_log(avctx, AV_LOG_FATAL, "Missing required palette via palctrl\n");
+        return -1;
+    }
     a->avctx = avctx;
     avctx->pix_fmt= PIX_FMT_PAL8;
     a->refdata = av_malloc(avctx->width * avctx->height);