REGRESSION(308123@main): [GTK] Crash in Clipboard::formats() when pasting with an empty clipboard - #77266

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REGRESSION(308123@main): [GTK] Crash in Clipboard::formats() when pasting with an empty clipboard
https://bugs.webkit.org/show_bug.cgi?id=324466

Reviewed by NOBODY (OOPS!).

When the targets cannot be retrieved, for example when the clipboard has no
owner, GTK 3 calls the targets callback with atoms == nullptr and n_atoms ==
-1. The loop over the atoms ran zero times for that until 308123@main turned
it into a span, which takes -1 as a huge size over a null pointer: the UI
process crashed on every paste of an empty clipboard. Clamp the count to
zero, as readBuffer() already does for its length.

* Source/WebKit/UIProcess/gtk/ClipboardGtk3.cpp:
(WebKit::Clipboard::formats):

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