patch 8.1.1707: Coverity warns for possibly using a NULL pointer

Problem:    Coverity warns for possibly using a NULL pointer.
Solution:   Change the logic to make sure no NULL pointer is used.

Author: brammool

src/popupwin.c

@@ -587,14 +587,13 @@ apply_general_options(win_T *wp, dict_T *dict)
     di = dict_find(dict, (char_u *)"mask", -1);
     if (di != NULL)
     {
-	int ok = TRUE;
+	int ok = FALSE;
 
-	if (di->di_tv.v_type != VAR_LIST)
-	    ok = FALSE;
-	else if (di->di_tv.vval.v_list != NULL)
+	if (di->di_tv.v_type == VAR_LIST && di->di_tv.vval.v_list != NULL)
 	{
 	    listitem_T *li;
 
+	    ok = TRUE;
 	    for (li = di->di_tv.vval.v_list->lv_first; li != NULL;
 							      li = li->li_next)
 	    {

src/testdir/test_popupwin.vim

@@ -643,6 +643,10 @@ func Test_popup_invalid_arguments()
   call popup_clear()
   call assert_fails('call popup_create([#{text: "text", props: ["none"]}], {})', 'E715:')
   call popup_clear()
+  call assert_fails('call popup_create("text", #{mask: ["asdf"]})', 'E475:')
+  call popup_clear()
+  call assert_fails('call popup_create("text", #{mask: test_null_list()})', 'E475:')
+  call popup_clear()
 endfunc
 
 func Test_win_execute_closing_curwin()

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1707,
 /**/
     1706,
 /**/