Terminal::Dispatcher::dispatch: Initialize map

Fixes this warning (which becomes an error with -Werror):

terminaldispatcher.cpp: In member function ‘void Terminal::Dispatcher::dispatch(Terminal::Function_Type, Parser::Action*, Terminal::Framebuffer*)’:
terminaldispatcher.cpp:156:19: error: ‘map’ may be used uninitialized in this function

Signed-off-by: Anders Kaseorg <andersk@mit.edu>