* ./src/playlist/playlist.c, src/input/input.c: added safety checks to
    prevent crashes on next file.
(ported from 0_4_1_branch)
  * ./plugins/gtk/gtk_display.c, ./plugins/win32/mainframe.cpp: we deactivate
    popup menus when no stream is being played, even in network mode.
  * ./src/input/mpeg_system.c: removed unnecessarily verbose message.