cmd/coordinator: verify snapshot before using it

We were creating corrupt snapshots before (golang/go#12671), so
double-check them before use, for now. We can relax this in the future
(see TODO).

Fixes golang/go#12671

Change-Id: I10b7929d4b78c717f29c67a0ce495ecd435a1d26
Reviewed-on: https://go-review.googlesource.com/14772
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>