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\\'

x.c

@@ -818,7 +818,7 @@ xloadcols(void)
 int
 xgetcolor(int x, unsigned char *r, unsigned char *g, unsigned char *b)
 {
-	if (!BETWEEN(x, 0, dc.collen))
+	if (!BETWEEN(x, 0, dc.collen - 1))
 		return 1;
 
 	*r = dc.col[x].color.red >> 8;
@@ -833,7 +833,7 @@ xsetcolorname(int x, const char *name)
 {
 	Color ncolor;
 
-	if (!BETWEEN(x, 0, dc.collen))
+	if (!BETWEEN(x, 0, dc.collen - 1))
 		return 1;
 
 	if (!xloadcolor(x, name, &ncolor))