patch 8.0.1842: popup menu inside terminal window isn't cleared

Problem:    Popup menu inside terminal window isn't cleared.
Solution:   Use NOT_VALID in pum_undisplay(). (suggested by Christian
            Brabandt, closes #2908)

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1842,
 /**/
     1841,
 /**/