gscan: catch potential ZeroDivisionError

[matthewrmshin]

A user reported this:

Suppose someone decided to open cylc gscan,
and then played around with the size of the window and perhaps did some too
drastic changes resulting in error messages something like

   cell_index = ((cell_x - dot_offset) // dot_width) + 1
ZeroDivisionError: integer division or modulo by zero

[matthewrmshin]

@oliver-sanders please feel free to assign a 2nd reviewer.

[oliver-sanders]

See also #2029