Do not match snippets that have locales if client locale is invalid.

Fixes a bug where invalid locales are matching tons of snippets that
they shouldn't be receiving.

See <a href="https://snippets.mozilla.com/3/Firefox/15.0.1/20120907230839/Linux_x86-gcc3/qcv/release/Linux 2.6.32-43-generic (GTK 2.20.1)/canonical/1.0/">this link</a> for an example of the bug happening on production.