Added range checks when initializing textinfo.screenwidth and screeheight

(don't wrap around if value is > 255) - presumably these textinfo struct
members were left as unsigned chars (8-bits) to preserve compatiblity with
legacy conio apps, but I'm not sure. Perhaps we could increase these to
at least 16-bit integers or bigger in the future, in which case these range
checks would need to be removed or replaced.