avcodec/mjpegdec: check bits per pixel for changes similar to dimensions

Fixes out of array accesses
Fixes: asan_heap-oob_16668e9_2_asan_heap-oob_16668e9_346_miss_congeniality_pegasus_mjpg.avi

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -244,7 +244,7 @@ int ff_mjpeg_decode_dht(MJpegDecodeContext *s)
 
 int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
 {
-    int len, nb_components, i, width, height, pix_fmt_id, ret;
+    int len, nb_components, i, width, height, bits, pix_fmt_id, ret;
     int h_count[MAX_COMPONENTS];
     int v_count[MAX_COMPONENTS];
 
@@ -254,11 +254,11 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
     /* XXX: verify len field validity */
     len     = get_bits(&s->gb, 16);
     s->avctx->bits_per_raw_sample =
-    s->bits = get_bits(&s->gb, 8);
+    bits = get_bits(&s->gb, 8);
 
     if (s->pegasus_rct)
-        s->bits = 9;
-    if (s->bits == 9 && !s->pegasus_rct)
+        bits = 9;
+    if (bits == 9 && !s->pegasus_rct)
         s->rct  = 1;    // FIXME ugly
 
     if(s->lossless && s->avctx->lowres){
@@ -291,7 +291,7 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
             return AVERROR_INVALIDDATA;
         }
     }
-    if (s->ls && !(s->bits <= 8 || nb_components == 1)) {
+    if (s->ls && !(bits <= 8 || nb_components == 1)) {
         avpriv_report_missing_feature(s->avctx,
                                       "JPEG-LS that is not <= 8 "
                                       "bits/component or 16-bit gray");
@@ -337,11 +337,13 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
 
     /* if different size, realloc/alloc picture */
     if (   width != s->width || height != s->height
+        || bits != s->bits
         || memcmp(s->h_count, h_count, sizeof(h_count))
         || memcmp(s->v_count, v_count, sizeof(v_count))) {
 
         s->width      = width;
         s->height     = height;
+        s->bits       = bits;
         memcpy(s->h_count, h_count, sizeof(h_count));
         memcpy(s->v_count, v_count, sizeof(v_count));
         s->interlaced = 0;