patch 9.0.0213: using freed memory with error in assert argument

Problem:    Using freed memory with error in assert argument.
Solution:   Make a copy of the error.

src/testdir/test_assert.vim

@@ -291,6 +291,10 @@ func Test_assert_fail_fails()
     let exp = v:exception
   endtry
   call assert_match("E1174: String required for argument 5", exp)
+
+  call assert_equal(1, assert_fails('c0', ['', '\1']))
+  call assert_match("Expected '\\\\\\\\1' but got 'E939: Positive count required: c0': c0", v:errors[0])
+  call remove(v:errors, 0)
 endfunc
 
 func Test_assert_fails_in_try_block()

src/testing.c

@@ -597,6 +597,7 @@ f_assert_fails(typval_T *argvars, typval_T *rettv)
     int		save_trylevel = trylevel;
     int		called_emsg_before = called_emsg;
     char	*wrong_arg_msg = NULL;
+    char_u	*tofree = NULL;
 
     if (check_for_string_or_number_arg(argvars, 0) == FAIL
 	    || check_for_opt_string_or_list_arg(argvars, 1) == FAIL
@@ -659,9 +660,12 @@ f_assert_fails(typval_T *argvars, typval_T *rettv)
 		expected_str = expected;
 	    }
 	    else if (list->lv_len == 2)
+	    {
+		// make a copy, an error in pattern_match() may free it
+		tofree = actual = vim_strsave(get_vim_var_str(VV_ERRMSG));
+		if (actual != NULL)
 		{
 		    tv = &list->lv_u.mat.lv_last->li_tv;
-		actual = get_vim_var_str(VV_ERRMSG);
 		    expected = tv_get_string_buf_chk(tv, buf);
 		    if (!pattern_match(expected, actual, FALSE))
 		    {
@@ -670,6 +674,7 @@ f_assert_fails(typval_T *argvars, typval_T *rettv)
 		    }
 		}
 	    }
+	}
 	else
 	{
 	    wrong_arg_msg = e_assert_fails_second_arg;
@@ -749,6 +754,7 @@ theend:
     msg_scrolled = 0;
     lines_left = Rows;
     VIM_CLEAR(emsg_assert_fails_msg);
+    vim_free(tofree);
     set_vim_var_string(VV_ERRMSG, NULL, 0);
     if (wrong_arg_msg != NULL)
 	emsg(_(wrong_arg_msg));

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    213,
 /**/
     212,
 /**/