Fix some const.

modules/demux/mkv.cpp

@@ -1444,7 +1444,7 @@ static int Open( vlc_object_t * p_this )
     demux_sys_t        *p_sys;
     matroska_stream_c  *p_stream;
     matroska_segment_c *p_segment;
-    uint8_t            *p_peek;
+    const uint8_t      *p_peek;
     std::string         s_path, s_filename;
     vlc_stream_io_callback *p_io_callback;
     EbmlStream         *p_io_stream;
@@ -1544,9 +1544,11 @@ static int Open( vlc_object_t * p_this )
 #endif
                         {
                             // test wether this file belongs to our family
-                            uint8_t *p_peek;
+                            const uint8_t *p_peek;
                             bool          file_ok = false;
-                            stream_t *p_file_stream = stream_UrlNew( p_demux, s_filename.c_str());
+                            stream_t      *p_file_stream = stream_UrlNew( 
+                                                            p_demux, 
+                                                            s_filename.c_str());
                             /* peek the begining */
                             if( p_file_stream &&
                                 stream_Peek( p_file_stream, &p_peek, 4 ) >= 4

modules/demux/mp4/mp4.c

@@ -268,7 +268,7 @@ static int Open( vlc_object_t * p_this )
     demux_t  *p_demux = (demux_t *)p_this;
     demux_sys_t     *p_sys;
 
-    uint8_t         *p_peek;
+    const uint8_t   *p_peek;
 
     MP4_Box_t       *p_ftyp;
     MP4_Box_t       *p_rmra;