mjpegdec: extend check for incompatible values of s->rgb and s->ls

This can happen if s->ls changes from 0 to 1, but picture allocation is
skipped due to s->interlaced.

In that case ff_jpegls_decode_picture could be called even though the
s->picture_ptr frame has the wrong pixel format and thus a wrong
linesize, which results in a too small zero buffer being allocated.

This fixes an out-of-bounds read in ls_decode_line.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/mjpegdec.c

@@ -632,7 +632,8 @@ unk_pixfmt:
         av_log(s->avctx, AV_LOG_DEBUG, "decode_sof0: error, len(%d) mismatch\n", len);
     }
 
-    if (s->rgb && !s->lossless && !s->ls) {
+    if ((s->rgb && !s->lossless && !s->ls) ||
+        (!s->rgb && s->ls && s->nb_components > 1)) {
         av_log(s->avctx, AV_LOG_ERROR, "Unsupported coding and pixel format combination\n");
         return AVERROR_PATCHWELCOME;
     }