mjpegdec: properly report unsupported disabled features

When JPEG-LS support is disabled the decoder would feed the
data to the JPEG Lossless decode_*_scan function resulting in
faulty decoding.

CC: libav-stable@libav.org

libavcodec/mjpegdec.c

@@ -1485,6 +1485,12 @@ int ff_mjpeg_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
             else if (start_code == COM)
                 mjpeg_decode_com(s);
 
+            if (!CONFIG_JPEGLS_DECODER &&
+                (start_code == SOF48 || start_code == LSE)) {
+                av_log(avctx, AV_LOG_ERROR, "JPEG-LS support not enabled.\n");
+                return AVERROR(ENOSYS);
+            }
+
             switch (start_code) {
             case SOI:
                 s->restart_interval = 0;