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'.

src/testdir/test_options.vim

@@ -319,3 +319,16 @@ func Test_set_values()
     throw 'Skipped: opt_test.vim does not exist'
   endif
 endfunc
+
+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