cc: fix an out of bounds array access

cc: fix an out of bounds array access

Alternatively, we could expand the ewidth array
in [568]c/txt.c to have NALLTYPES elements and
give all types above NTYPE a width of -1.

I don't think it's worth it since TDOT and TOLD
are the only two type values above NTYPE that
are passed to typ:

$ /tmp/cctypes
cc/dcl.c:683: 			t->down = typ(TOLD, T);
cc/dcl.c:919: 		return typ(TDOT, T);
$

Fixes #3063.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5694047