[Reviewer: Alex] Check that an element exists after <Method>REGISTER</Method> in IFCs (fixes #356)

Alex,

Please can you review my fix to check that we have a node after `<Method>REGISTER</Method>` before checking that it is an `<Extension>`, when parsing IFCs?  I've repro-ed this in UT (see below) and it fixes the issue.  A quick turn-around would be appreciated - I'd like to build a patch tonight.

Matt