replace magic calcuation with one based
on the size of the array.  -1 to account for
the trailing zero element, which is clearly
not a real register.

Notes:
Fri Jan 4 17:27:37 EST 2013 geoff
    already done differently.


Reference: /n/sources/patch/sorry/tblbad
Date: Fri Jan  4 17:02:41 CET 2013
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/src/cmd/tbl/tr.c	Fri Jan  4 17:02:11 2013
+++ /sys/src/cmd/tbl/tr.c	Fri Jan  4 17:02:10 2013
@@ -20,7 +20,7 @@
 char	*
 reg(int col, int place)
 {
-	if (sizeof(nregs) < 2 * 3 * qcol)
+	if(qcol*place+col >= nelem(nregs)-1)
 		error("Too many columns for registers");
 	return (nregs[qcol*place+col]);
 }