ensure nav/win are false if navigator/window undefined

[agallant]

I was working on cutting new releases and found that core integration tests were failing because the environment was referring to undefined variables. This tweak fixes that, and then I'll cut a release here and in the firefox and chrome flavors.

For the flavors, I think the only thing to do is add the provider to entry (https://github.com/freedomjs/freedom-for-firefox/commit/129bab93e2b3089ee533e61cb057555fbfd5e1d7, https://github.com/freedomjs/freedom-for-chrome/commit/98aa7a230092bf21d7c2507fae7a3fac912a7442) and bump the freedom version dependency as appropriate, as this is a fairly simple provider. Let me know if I'm missing something though - thanks!

[coveralls]

Coverage remained the same at 81.17% when pulling 616147a0573b5974b45a66546604a74b68c7d636 on soycode-fixcoreonline into 069c4ddc131a77cf411c40a50b7bd6344c9a9b2b on master.

[bemasc]

:+1: