cocoa-cb: fix crash when no screen is available

instead of force unwrapping and chaining the optional vars in our
containsMouseLocation function, safely unwrap and guard the resulting
var.

Fixes #6062

(cherry picked from commit 6bf0edc59c)

video/out/cocoa-cb/events_view.swift

@@ -233,7 +233,7 @@ class EventsView: NSView {
             topMargin = cocoaCB.window.titleBarHeight + 1 + menuBarHeight
         }
 
-        var vF = window!.screen!.frame
+        guard var vF = window?.screen?.frame else { return false }
         vF.size.height -= topMargin
 
         let vFW = window!.convertFromScreen(vF)