patch 8.1.1665: crash when popup window with mask is below the screen

Problem:    Crash when popup window with mask is below the screen.
Solution:   Correct boundary check.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 4e3170c..104ebd5 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -486,6 +486,10 @@
   call term_sendkeys(buf, ":call popup_move(winidb, {'pos': 'topright', 'col': 12})\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_mask_4', {})
 
+  call term_sendkeys(buf, ":call popup_move(winid, {'pos': 'topright', 'col': 12, 'line': 11})\<CR>")
+  call term_sendkeys(buf, ":call popup_move(winidb, {'pos': 'topleft', 'col': 42, 'line': 11})\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_mask_5', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestPopupMask')