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)
diff --git a/src/version.c b/src/version.c
index 67a6acf..af967d1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1842,
+/**/
     1841,
 /**/
     1840,