patch 8.0.0582: illegal memory access with z= command

Problem:    Illegal memory access with z= command. (Dominique Pelle)
Solution:   Avoid case folded text to be longer than the original text.  Use
            MB_PTR2LEN() instead of MB_BYTE2LEN().

src/testdir/test_spell.vim

@@ -18,3 +18,12 @@ func Test_wrap_search()
   bwipe!
   set nospell
 endfunc
+
+func Test_z_equal_on_invalid_utf8_word()
+  split
+  set spell
+  call setline(1, "\xff")
+  norm z=
+  set nospell
+  bwipe!
+endfunc