plot.antsImage bug: error for empty overlay

[dorianps]

Sometimes the overlay image might be an empty image. When plot is attempted in that case, there is an error.

> img = antsImageRead('ch2.nii.gz')
> imgempty = img*0
> plot(img,imgempty)
Error in if (window.overlay[1] == window.overlay[2]) { :
  missing value where TRUE/FALSE needed

Trying to set window.overlay gives another error.

> plot(img,imgempty, window.overlay=c(0,0))
Error in max(c(zeroval, (zeroval))):(zeroval + 1) : NA/NaN argument
> plot(img,imgempty, window.overlay=c(0,1))
Error in max(c(zeroval, (zeroval))):(zeroval + 1) : NA/NaN argument

I know which lines are causing the error, but don't have time right now to follow the function flow and apply a surgical fix. Keeping this issue open as a reminder to anyone who can fix the bug.