keygen/keygen.go: fix Close
keygen/keygen.go: fix Close

Change-Id: If3c3e2419b860ec84e088455460366e152186ad1
Reviewed-on: https://upspin-review.git.corp.google.com/1287
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin