vim-patch:8.0.0575: using freed memory when resetting 'indentexpr'

Problem:    Using freed memory when resetting 'indentexpr' while evaluating
            it. (Dominique Pelle)
Solution:   Make a copy of 'indentexpr'.
a701b3b6f0

src/nvim/indent.c

@@ -538,7 +538,14 @@ int get_expr_indent(void)
     sandbox++;
   }
   textlock++;
-  indent = (int)eval_to_number(curbuf->b_p_inde);
+
+  // Need to make a copy, the 'indentexpr' option could be changed while
+  // evaluating it.
+  char_u *inde_copy = vim_strsave(curbuf->b_p_inde);
+  if (inde_copy != NULL) {
+    indent = (int)eval_to_number(inde_copy);
+    xfree(inde_copy);
+  }
 
   if (use_sandbox) {
     sandbox--;

src/nvim/testdir/test_options.vim

@@ -275,3 +275,15 @@ func Test_complete()
   set complete&
 endfun
 
+func ResetIndentexpr()
+  set indentexpr=
+endfunc
+
+func Test_set_indentexpr()
+  " this was causing usage of freed memory
+  set indentexpr=ResetIndentexpr()
+  new
+  call feedkeys("i\<c-f>", 'x')
+  call assert_equal('', &indentexpr)
+  bwipe!
+endfunc