Fix bug in singfun.plus().
Fix bug in singfun.plus().

Three things wrong here:

1.  if (f) doesn't work if f is complex because MATLAB doesn't convert
complex-valued scalars to logicals.

2.  Assuming it did work, if (f) should be if (~f), since these cases should
trigger when f is zero.

3.  The second comment needed F and G interchanged.

asgeirbirkis/chebfun,ilovejs/chebfun,alshedivat/chebfun