check loginCallbackId exists before pluginResult call

We make sure that loginCallbackId exists before calling it. When it's not set, it triggers the exception referenced in issue #507
Update FacebookConnectPlugin.m

Tweaked the PRed code to be more inline with Google Objective-C style guide.
Merge pull request #1 from aogilvie/patch-2

Update FacebookConnectPlugin.m
Merge pull request #510 from jakecraige/patch-1

check loginCallbackId exists before pluginResult call