Fix segfault on SDL event without window

Since #5804, controls have been enabled even with --no-window. As a
result, the Android clipboard is synchronized with the computer, causing
SDL to trigger an SDL_CLIPBOARDUPDATE event.

This event is ignored by scrcpy, but it was still transmitted to the
sc_screen instance, even if it had not been initialized.

Fix the issue by calling sc_screen_handle_event() only when a screen
instance exists.

Refs #5804 <https://github.com/Genymobile/scrcpy/pull/5804>
Fixes #5970 <https://github.com/Genymobile/scrcpy/issues/5970>