cmd/cc: fix typo leading to index out of range.
cmd/cc: fix typo leading to index out of range.

Detected by GCC static analysis.

Fixes issue 5117.

R=golang-dev, ality, minux.ma
CC=golang-dev
https://codereview.appspot.com/7665047
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go