patch 8.2.2113: MS-Windows GUI: crash after using ":set guifont=" four times

Problem:    MS-Windows GUI: crash after using ":set guifont=" four times.
Solution:   Check for NULL pointer. (Ken Takata, closes #7434)

src/gui_dwrite.cpp

@@ -239,6 +239,7 @@ public:
 	if (mItems[n].pTextFormat != item.pTextFormat)
 	{
 	    SafeRelease(&mItems[n].pTextFormat);
+	    if (item.pTextFormat != NULL)
 		item.pTextFormat->AddRef();
 	}
 	mItems[n] = item;

src/testdir/test_gui.vim

@@ -386,6 +386,13 @@ func Test_set_guifont()
   if has('win32')
     " Invalid font names are accepted in GTK GUI
     call assert_fails('set guifont=xa1bc23d7f', 'E596:')
+
+    " doing this four times used to cause a crash
+    set guifont=
+    set guifont=
+    set guifont=
+    set guifont=
+    set guifont=
   endif
 
   if has('xfontset')

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2113,
 /**/
     2112,
 /**/