Avoid double free in case of corrupted files

(cherry picked from commit 6cca95fb3950f4e2a1cdb2cfa2bc96538dd897a6)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/Ebml_parser.cpp

@@ -121,6 +121,11 @@ void EbmlParser::Keep( void )
     mb_keep = true;
 }
 
+void EbmlParser::Unkeep()
+{
+    mb_keep = false;
+}
+
 int EbmlParser::GetLevel( void ) const
 {
     return mi_user_level;

modules/demux/mkv/Ebml_parser.hpp

@@ -41,6 +41,7 @@ class EbmlParser
     void Reset( demux_t *p_demux );
     EbmlElement *Get( int n_call = 0 );
     void        Keep( void );
+    void        Unkeep( void );
     EbmlElement *UnGet( uint64 i_block_pos, uint64 i_cluster_pos );
 
     int  GetLevel( void ) const;

modules/demux/mkv/matroska_segment.cpp

@@ -1502,7 +1502,7 @@ int matroska_segment_c::BlockGet( KaxBlock * & pp_block, KaxSimpleBlock * & pp_s
             /* Check blocks validity to protect againts broken files */
             if( BlockFindTrackIndex( NULL, pp_block , pp_simpleblock ) )
             {
-                delete pp_block;
+                ep->Unkeep();
                 pp_simpleblock = NULL;
                 pp_block = NULL;
                 continue;
@@ -1625,6 +1625,9 @@ int matroska_segment_c::BlockGet( KaxBlock * & pp_block, KaxSimpleBlock * & pp_s
                     {
                         msg_Err( &sys.demuxer, "Error while reading %s... upping level", typeid(*el).name());
                         ep->Up();
+                        ep->Unkeep();
+                        pp_simpleblock = NULL;
+                        pp_block = NULL;
                         break;
                     }
                     if( MKV_IS_ID( el, KaxBlock ) )
@@ -1678,6 +1681,9 @@ int matroska_segment_c::BlockGet( KaxBlock * & pp_block, KaxSimpleBlock * & pp_s
         {
             msg_Err( &sys.demuxer, "Error while reading %s... upping level", typeid(*el).name());
             ep->Up();
+            ep->Unkeep();
+            pp_simpleblock = NULL;
+            pp_block = NULL;
         }
     }
 }