GStreamerSharp: protect call to event delegates

This is a good practice to avoid potential race conditions, that
is already used in other events, and is best explained in:

http://www.mono-project.com/Gendarme.Rules.Concurrency#ProtectCallToEventDelegatesRule

or

https://github.com/spouliot/gendarme/wiki/Gendarme.Rules.Concurrency.ProtectCallToEventDelegatesRule%282.10%29