Crash on window resize (SIGWINCH) (Unix)

[srlehn]

This is fixed by #15:

This difers from the already fixed #17 which happens instantly! see #17 (comment) for details.

When the window is resized a lot it eventually crashes with "panic: send on closed channel"

tested with termui from gizak/termui#233

test go code

[srlehn]

#15 is merged