Avoid infinite loop if eof happens during buffering

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit b1788573)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/mkv.cpp

@@ -668,7 +668,7 @@ static int Demux( demux_t *p_demux)
         if( p_sys->i_pts >= p_sys->i_start_pts  )
             if ( p_vsegment->UpdateCurrentToChapter( *p_demux ) )
             {
-                i_return = 1;
+                i_return = ( i_block_count != 0 );
                 break;
             }
 
@@ -694,7 +694,7 @@ static int Demux( demux_t *p_demux)
                     /* TODO handle successive chapters with the same user_start_time/user_end_time
                     */
                     p_sys->i_pts = p_chap->i_virtual_stop_time;
-                    p_sys->i_pts++; // trick to avoid staying on segments with no duration and no content
+                    p_sys->i_start_pts =  p_sys->i_pts++; // trick to avoid staying on segments with no duration and no content
 
                     i_return = 1;
                 }