hotplug_exit: mutex protection of context device list while cleaning it

This prevents concurrent access to the list by the event background
thread, which could still be processing a previous hotplug event and
having to modify the list

libusb/hotplug.c

@@ -213,6 +213,7 @@ void usbi_hotplug_exit(struct libusb_context *ctx)
 	}
 
 	/* free all discovered devices. due to parent references loop until no devices are freed. */
+	usbi_mutex_lock(&ctx->usb_devs_lock); /* hotplug thread might still be processing an already triggered event, possibly accessing this list as well */
 	for_each_device_safe(ctx, dev, next_dev) {
 		/* remove the device from the usb_devs list only if there are no
 		 * references held, otherwise leave it on the list so that a
@@ -225,6 +226,7 @@ void usbi_hotplug_exit(struct libusb_context *ctx)
 
 		libusb_unref_device(dev);
 	}
+	usbi_mutex_unlock(&ctx->usb_devs_lock);
 
 	usbi_mutex_destroy(&ctx->hotplug_cbs_lock);
 }