gui: Fix off-by-one in add_spaces().

gui: Fix off-by-one in add_spaces().

Commit e90c6c0a (Speed up add_spaces().) changed add_spaces() to print
space characters in chunks rather than one at a time. It introduced
space[], an array of whitespace characters, which is written entirely
if there are more spaces to print than the size of the array. However,
in the calculation of how much was printed so far, we missed the fact
that sizeof(space) includes the terminating NULL byte, so this number
is in fact the number of space characters *plus one*.

Consequently, we printed too few space characters. This resulted in
parts of the previous string still being visible in the top window
of para_gui.