desktop-shell: End popup when starting a touch shell grab

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=77345
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index 79bd65c..e4c0295 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -364,6 +364,9 @@
 {
 	struct desktop_shell *shell = shsurf->shell;
 
+	if (touch->seat->pointer)
+		popup_grab_end(touch->seat->pointer);
+
 	grab->grab.interface = interface;
 	grab->shsurf = shsurf;
 	grab->shsurf_destroy_listener.notify = destroy_shell_grab_shsurf;