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.

Fixes #1366
Fixes #1445
Closes #1452
References #1406

libusb/hotplug.c

@@ -214,6 +214,7 @@ void usbi_hotplug_exit(struct libusb_context *ctx)
  free(msg);
  }
 
+ usbi_mutex_lock(&ctx->usb_devs_lock); /* hotplug thread might still be processing an already triggered event, possibly accessing this list as well */
  /* free all discovered devices */
  for_each_device_safe(ctx, dev, next_dev) {
  /* remove the device from the usb_devs list only if there are no
@@ -227,6 +228,7 @@ 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);
 }

libusb/version_nano.h

@@ -1 +1 @@
-#define LIBUSB_NANO 11885
+#define LIBUSB_NANO 11886