Fix rt-safety of LuaProc w/o chanmapping

Since Lua function arguments are not typed, there is no
explicit "const", and a function can always modify the parameter.

When passing `ChanMapping const&` as argument, the object is
copy constructed. In this specific case the std::map<> members
of ChanMapping allocate memory.

Passing a pointer to the object works around this issue.

LuaBridge later dereferences the object as needed when calling
c++ methods, and copy-construction would only happen later.

libs/ardour/luaproc.cc

@@ -651,7 +651,7 @@ LuaProc::connect_and_run (BufferSet& bufs,
 	try {
 		if (_lua_does_channelmapping) {
 			// run the DSP function
-			(*_lua_dsp)(&bufs, in, out, nframes, offset);
+			(*_lua_dsp)(&bufs, &in, &out, nframes, offset);
 		} else {
 			// map buffers
 			BufferSet& silent_bufs  = _session.get_silent_buffers (ChanCount (DataType::AUDIO, 1));