[CoreMidi] Fix MidiEventList cleanup cast.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rolfbjarne

src/CoreMidi/MidiEventList.cs

@@ -91,7 +91,7 @@ public MidiEventList (MidiProtocolId protocol, int size)
 				midiDataPointer = (MIDIEventList*) Marshal.AllocHGlobal (midiDataSize);
 				currentPacket = MIDIEventListInit (midiDataPointer, protocol);
 				if (currentPacket is null) {
-					Marshal.FreeHGlobal (midiDataPointer);
+					Marshal.FreeHGlobal ((IntPtr) midiDataPointer);
 					midiDataPointer = null;
 					throw new Exception ($"Failed to create midi event list.");
 				}