getKeysForFuncProxy should always return a collection, because of where it's used:

https://github.com/supercollider/supercollider/blob/master/SCClassLibrary/Common/Control/ResponseDefs.sc#L131-L132

		keys = this.getKeysForFuncProxy(funcProxy);
		keys.do({|key| active[key] = active[key].addFunc(func) }); // support multiple keys

All of the other MIDI***Dispatcher classes do asArray in getKeysForFuncProxy... MIDISysexDispatcher, uniquely, does not:

	getKeysForFuncProxy {|funcProxy| ^(funcProxy.srcID ? \all)}

So, when the srcID is a very large integer, keys becomes a large integer, and then keys.do will add a new item into active that number of times. Millions of times. Potentially-crashy numbers of times.

Fixes #2937.