Fix check for empty colors left.
Because pair "0" is reserved (and we start searching at index 1), we
only have access to (COLOR_PAIRS - 1) pairs.
Also, don't increment UserColors during the check, because that would
incorrectly inflate the number of color pairs allocated if none are
available. That would delay or even remove the ability to reallocate
pairs after future color pair frees.
It actually would make more sense to just check (index > COLOR_PAIRS -
1). But since UserColors is incremented here, and is used in dprint
statements, keep the check consistent.