Bug 843736 - [sms] Application broken when running with FF18

In FireFox 18, the global `navigator` object has a `mozSettings`
attribute, but it is set to `null`. In order to properly test with
FireFox, code that attempts to reference properties of
`navigator.mozSettings` should check that this value is non-null.

r=kgrandon
Merge pull request #8245 from jugglinmike/fix-sms-on-ff18

Bug 843736 - [sms] Application broken when running with FF18 r=kgrandon