patch 8.1.1615: crash when passing buffer number to popup_create()

Problem:    Crash when passing buffer number to popup_create(). (Yasuhiro
            Matsumoto)
Solution:   Initialze the window properly.

src/popupwin.c

@@ -1056,8 +1056,7 @@ popup_create(typval_T *argvars, typval_T *rettv, create_type_T type)
     {
 	// use existing buffer
 	new_buffer = FALSE;
-	wp->w_buffer = buf;
-	++buf->b_nwindows;
+	win_init_popup_win(wp, buf);
 	buffer_ensure_loaded(buf);
     }
     else

src/testdir/test_popupwin.vim

@@ -1668,4 +1668,9 @@ func Test_popupwin_with_buffer()
   call assert_equal({}, popup_getpos(winid))
   call assert_equal(1, bufloaded(buf))
   exe 'bwipe! ' .. buf
+
+  edit test_popupwin.vim
+  let winid = popup_create(bufnr(''), {})
+  redraw
+  call popup_close(winid)
 endfunc

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1615,
 /**/
     1614,
 /**/