vim-patch:8.0.1433: illegal memory access after undo

Problem:    Illegal memory access after undo. (Dominique Pelle)
Solution:   Avoid the column becomes negative. (Christian Brabandt,
            closes vim/vim#2533)
95dbcbea6d

src/nvim/mark.c

@@ -1465,7 +1465,11 @@ void mark_mb_adjustpos(buf_T *buf, pos_T *lp)
 {
   if (lp->col > 0 || lp->coladd > 1) {
     const char_u *const p = ml_get_buf(buf, lp->lnum, false);
-    lp->col -= (*mb_head_off)(p, p + lp->col);
+    if (*p == NUL || (int)STRLEN(p) < lp->col) {
+      lp->col = 0;
+    } else {
+      lp->col -= (*mb_head_off)(p, p + lp->col);
+    }
     // Reset "coladd" when the cursor would be on the right half of a
     // double-wide character.
     if (lp->coladd == 1

src/nvim/mbyte.c

@@ -566,7 +566,9 @@ int utf_off2cells(unsigned off, unsigned max_off)
 /// Convert a UTF-8 byte sequence to a wide character
 ///
 /// If the sequence is illegal or truncated by a NUL then the first byte is
-/// returned. Does not include composing characters for obvious reasons.
+/// returned.
+/// For an overlong sequence this may return zero.
+/// Does not include composing characters for obvious reasons.
 ///
 /// @param[in]  p  String to convert.
 ///

src/nvim/testdir/test_undo.vim

@@ -348,3 +348,12 @@ func Test_redo_empty_line()
   exe "norm."
   bwipe!
 endfunc
+
+" This used to cause an illegal memory access
+func Test_undo_append()
+  new
+  call feedkeys("axx\<Esc>v", 'xt')
+  undo
+  norm o
+  quit
+endfunc