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.

src/testdir/test_visual.vim

@@ -1339,5 +1339,20 @@ func Test_visual_ex_copy_line()
   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

src/undo.c

@@ -3029,6 +3029,8 @@ u_undo_end(
 	}
     }
 #endif
+    if (VIsual_active)
+	check_pos(curbuf, &VIsual);
 
     smsg_attr_keep(0, _("%ld %s; %s #%ld  %s"),
 	    u_oldcount < 0 ? -u_oldcount : u_oldcount,

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4217,
 /**/
     4216,
 /**/