device: fix crash when unplugging certain devices on macOS

Fixes #621

panda/src/device/ioKitInputDevice.cxx

@@ -22,8 +22,11 @@
 #include "mouseButton.h"
 
 static void removal_callback(void *ctx, IOReturn result, void *sender) {
-  IOKitInputDevice *input_device = (IOKitInputDevice *)ctx;
+  // We need to hold a reference to this because it may otherwise be destroyed
+  // during the call to on_remove().
+  PT(IOKitInputDevice) input_device = (IOKitInputDevice *)ctx;
   nassertv(input_device != nullptr);
+  nassertv(input_device->test_ref_count_integrity());
   input_device->on_remove();
 }