updater_state_unittests: don't lookup a nil map on non-Windows platforms.

UpdaterState::GetState returns nullptr, if the current platform is
not Windows, and then the test calls at() method on the returned map.

This fixes LTO Linux bot:
https://build.chromium.org/p/chromium.fyi/builders/LTO%20Linux/builds/1560

BUG=615187

Review-Url: https://codereview.chromium.org/2536723004
Cr-Commit-Position: refs/heads/master@{#434845}
1 file changed