Avoids global array buffer overflows if they are
indexed using some of the values between NTYPE
and NALLTYPE.  It is entirely likely that not all of these
are necessary, but this is the C compiler and not worth
worrying much about.  This change takes up only a
few more bytes of memory and makes the behavior
deterministic.

Fixes #3078.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5693052