vorbisdemux: Check private context in theoras gtopts.

This prevents a null ptr dereference.
It could be checked differently but this way it should
be possible to return some data.

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

libavformat/oggparseskeleton.c

@@ -74,8 +74,11 @@ static int skeleton_header(AVFormatContext *s, int idx)
         target_idx = ogg_find_stream(ogg, AV_RL32(buf+12));
         start_granule = AV_RL64(buf+36);
         if (target_idx >= 0 && start_granule != -1) {
+            int64_t pts = ogg_gptopts(s, target_idx, start_granule, NULL);
+            if (pts == AV_NOPTS_VALUE)
+                return -1;
             ogg->streams[target_idx].lastpts =
-            s->streams[target_idx]->start_time = ogg_gptopts(s, target_idx, start_granule, NULL);
+            s->streams[target_idx]->start_time = pts;
         }
     }
 

libavformat/oggparsetheora.c

@@ -131,8 +131,13 @@ theora_gptopts(AVFormatContext *ctx, int idx, uint64_t gp, int64_t *dts)
     struct ogg *ogg = ctx->priv_data;
     struct ogg_stream *os = ogg->streams + idx;
     struct theora_params *thp = os->private;
-    uint64_t iframe = gp >> thp->gpshift;
-    uint64_t pframe = gp & thp->gpmask;
+    uint64_t iframe, pframe;
+
+    if (!thp)
+        return AV_NOPTS_VALUE;
+
+    iframe = gp >> thp->gpshift;
+    pframe = gp & thp->gpmask;
 
     if (thp->version < 0x030201)
         iframe++;