Added ability to externally retrieve linked IParam


This definitely returns a NULL pointer when the paramIdx is < 0. Although there are various comments in IPlug about using paramIdx > kNumParams in fact in my experience this is totally unsafe, and at present should be avoided. This is something I might take a look at fixing at a later time, as as far as I can see you can have non-automated params at the moment, but not params that are hidden entirely from any preset system, which is a useful facility.