patch 8.2.4217: illegal memory access when undo makes Visual area invalid

Problem:    Illegal memory access when undo makes Visual area invalid.
Solution:   Correct the Visual area after undo.
diff --git a/src/testdir/test_visual.vim b/src/testdir/test_visual.vim
index 9b322fd..b2beda0 100644
--- a/src/testdir/test_visual.vim
+++ b/src/testdir/test_visual.vim
@@ -1339,5 +1339,20 @@
   bwipe!
 endfunc
 
+" This was leaving the end of the Visual area beyond the end of a line.
+" Set 'undolevels' to start a new undo block.
+func Test_visual_undo_deletes_last_line()
+  new
+  call setline(1, ["aaa", "ccc", "dyd"])
+  set undolevels=100
+  exe "normal obbbbbbbbbxbb\<Esc>"
+  set undolevels=100
+  /y
+  exe "normal ggvjfxO"
+  undo
+  normal gNU
+  bwipe!
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/undo.c b/src/undo.c
index 4d186d4..636144a 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -3029,6 +3029,8 @@
 	}
     }
 #endif
+    if (VIsual_active)
+	check_pos(curbuf, &VIsual);
 
     smsg_attr_keep(0, _("%ld %s; %s #%ld  %s"),
 	    u_oldcount < 0 ? -u_oldcount : u_oldcount,
diff --git a/src/version.c b/src/version.c
index f5f9a2e..6685b55 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4217,
+/**/
     4216,
 /**/
     4215,