I'm improving gccgo's detection of variables that are only set
but not used, and it triggers additional errors on this code.
The new gccgo errors are correct; gc seems to suppress them
due to the other, expected, errors.  This change uses the
variables so that no compiler will complain.

gccgo change is https://golang.org/cl/119920043 .

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/116050043