mlpdec: Do not set invalid context in read_restart_header

The faulty values rippled further down the codepath causing a
hard-to-track segfault in the assembly code.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/mlpdec.c

@@ -389,9 +389,10 @@ static int read_restart_header(MLPDecodeContext *m, GetBitContext *gbp,
     uint8_t checksum;
     uint8_t lossless_check;
     int start_count = get_bits_count(gbp);
-    const int max_matrix_channel = m->avctx->codec_id == AV_CODEC_ID_MLP
-                                 ? MAX_MATRIX_CHANNEL_MLP
-                                 : MAX_MATRIX_CHANNEL_TRUEHD;
+    int min_channel, max_channel, max_matrix_channel;
+    const int std_max_matrix_channel = m->avctx->codec_id == AV_CODEC_ID_MLP
+                                     ? MAX_MATRIX_CHANNEL_MLP
+                                     : MAX_MATRIX_CHANNEL_TRUEHD;
 
     sync_word = get_bits(gbp, 13);
 
@@ -410,18 +411,18 @@ static int read_restart_header(MLPDecodeContext *m, GetBitContext *gbp,
 
     skip_bits(gbp, 16); /* Output timestamp */
 
-    s->min_channel        = get_bits(gbp, 4);
-    s->max_channel        = get_bits(gbp, 4);
-    s->max_matrix_channel = get_bits(gbp, 4);
+    min_channel        = get_bits(gbp, 4);
+    max_channel        = get_bits(gbp, 4);
+    max_matrix_channel = get_bits(gbp, 4);
 
-    if (s->max_matrix_channel > max_matrix_channel) {
+    if (max_matrix_channel > std_max_matrix_channel) {
         av_log(m->avctx, AV_LOG_ERROR,
                "Max matrix channel cannot be greater than %d.\n",
                max_matrix_channel);
         return AVERROR_INVALIDDATA;
     }
 
-    if (s->max_channel != s->max_matrix_channel) {
+    if (max_channel != max_matrix_channel) {
         av_log(m->avctx, AV_LOG_ERROR,
                "Max channel must be equal max matrix channel.\n");
         return AVERROR_INVALIDDATA;
@@ -437,12 +438,16 @@ static int read_restart_header(MLPDecodeContext *m, GetBitContext *gbp,
         return AVERROR_PATCHWELCOME;
     }
 
-    if (s->min_channel > s->max_channel) {
+    if (min_channel > max_channel) {
         av_log(m->avctx, AV_LOG_ERROR,
                "Substream min channel cannot be greater than max channel.\n");
         return AVERROR_INVALIDDATA;
     }
 
+    s->min_channel        = min_channel;
+    s->max_channel        = max_channel;
+    s->max_matrix_channel = max_matrix_channel;
+
 #if FF_API_REQUEST_CHANNELS
     if (m->avctx->request_channels > 0 &&
         m->avctx->request_channels <= s->max_channel + 1 &&