patch 8.2.2243: crash when popup mask contains zeroes

Problem:    Crash when popup mask contains zeroes.
Solution:   Check boundaries properly. (closes #7569)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index b8641fe..bfc0494 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -821,6 +821,10 @@
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestPopupMask')
+
+  " this was causing a crash
+  call popup_create('test', #{mask: [[0, 0, 0, 0]]})
+  call popup_clear()
 endfunc
 
 func Test_popup_select()