Fix bounds checks of dc.col

Fix bounds checks of dc.col

dc.collen is the length of dc.col, not the maximum index, hence if x is
equal to dc.collen, then it's an error.

With config.def.h, the last valid index is 259, so this correctly
reports "black":

    $ printf '\033]4;259;?\e\\'

260 is an invalid index and this reports garbage instead of printing an
error:

    $ printf '\033]4;260;?\e\\'