avcodec/mjpegdec: check that index is not negative

libavcodec/mjpegdec.c

@@ -373,7 +373,10 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
     s->v_max         = 1;
     for (i = 0; i < nb_components; i++) {
         /* component id */
-        s->component_id[i] = get_bits(&s->gb, 8) - 1;
+        int id = get_bits(&s->gb, 8);
+        if (id == 0)
+            return AVERROR_INVALIDDATA;
+        s->component_id[i] = id - 1;
         h_count[i]         = get_bits(&s->gb, 4);
         v_count[i]         = get_bits(&s->gb, 4);
         /* compute hmax and vmax (only used in interleaved case) */
@@ -1677,7 +1680,10 @@ int ff_mjpeg_decode_sos(MJpegDecodeContext *s, const uint8_t *mb_bitmask,
         return AVERROR_INVALIDDATA;
     }
     for (i = 0; i < nb_components; i++) {
-        id = get_bits(&s->gb, 8) - 1;
+        id = get_bits(&s->gb, 8);
+        if (id == 0)
+            return AVERROR_INVALIDDATA;
+        id -= 1;
         av_log(s->avctx, AV_LOG_DEBUG, "component: %d\n", id);
         /* find component index */
         for (index = 0; index < s->nb_components; index++)