Lua API: vis:pipe() causes a segfault when called before a window is open

[Randy Palamar]

Since commit 424b219 vis will segfault if vis:pipe() is called when
no windows are open. This can happen if you have some function calling
vis:pipe() based on an event such as FILE_OPEN which doesn't actually
depend on a window being present. For example my vis-gpg plugin does
this when decrypting a file.

A temporary patch will follow in a pull request.