lib: Add fpi_poll_is_setup() sanity check

This checks whether polling was correctly setup for integration with a
mainloop.

libfprint/fpi-poll.c

@@ -482,6 +482,12 @@ void fpi_poll_exit(void)
 	libusb_set_pollfd_notifiers(fpi_usb_ctx, NULL, NULL, NULL);
 }
 
+gboolean
+fpi_poll_is_setup(void)
+{
+	return (fd_added_cb != NULL && fd_removed_cb != NULL);
+}
+
 void
 fpi_timeout_cancel_all_for_dev(struct fp_dev *dev)
 {

libfprint/fpi-poll.h

@@ -48,4 +48,6 @@ void fpi_timeout_set_name(fpi_timeout *timeout,
 			  const char  *name);
 void fpi_timeout_cancel(fpi_timeout *timeout);
 
+gboolean fpi_poll_is_setup(void);
+
 #endif