hevc_ps: check overflow and test alternate syntax

Some streams were found to have what appeared to be truncated SPS.
Their syntax seem to be valid at least until the end of the VUI, so
try that syntax if the parsing would overflow the SPS in the
conforming syntax.

Fixes ticket #3872.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/hevc_ps.c

@@ -471,7 +471,8 @@ static void decode_vui(HEVCContext *s, HEVCSPS *sps)
 {
     VUI *vui          = &sps->vui;
     GetBitContext *gb = &s->HEVClc->gb;
-    int sar_present;
+    GetBitContext backup;
+    int sar_present, alt = 1;
 
     av_log(s->avctx, AV_LOG_DEBUG, "Decoding VUI\n");
 
@@ -525,6 +526,10 @@ static void decode_vui(HEVCContext *s, HEVCSPS *sps)
     vui->frame_field_info_present_flag = get_bits1(gb);
 
     vui->default_display_window_flag = get_bits1(gb);
+    // Backup context in case an alternate header is detected
+    if( get_bits_left(gb) >= 66)
+        memcpy(&backup, gb, sizeof(backup));
+
     if (vui->default_display_window_flag) {
         //TODO: * 2 is only valid for 420
         vui->def_disp_win.left_offset   = get_ue_golomb_long(gb) * 2;
@@ -552,8 +557,22 @@ static void decode_vui(HEVCContext *s, HEVCSPS *sps)
     vui->vui_timing_info_present_flag = get_bits1(gb);
 
     if (vui->vui_timing_info_present_flag) {
+        if( get_bits_left(gb) < 66) {
+            // The alternate syntax seem to have timing info located
+            // at where def_disp_win is normally located
+            av_log(s->avctx, AV_LOG_WARNING,
+                   "Strange VUI timing information, retrying...\n");
+            vui->default_display_window_flag = 0;
+            memset(&vui->def_disp_win, 0, sizeof(vui->def_disp_win));
+            memcpy(gb, &backup, sizeof(backup));
+            alt = 1;
+        }
         vui->vui_num_units_in_tick               = get_bits_long(gb, 32);
         vui->vui_time_scale                      = get_bits_long(gb, 32);
+        if (alt) {
+            av_log(s->avctx, AV_LOG_INFO, "Retry got %i/%i fps\n",
+                   vui->vui_time_scale, vui->vui_num_units_in_tick);
+        }
         vui->vui_poc_proportional_to_timing_flag = get_bits1(gb);
         if (vui->vui_poc_proportional_to_timing_flag)
             vui->vui_num_ticks_poc_diff_one_minus1 = get_ue_golomb_long(gb);