REGRESSION(274689@main): [GTK][Debug] Triggers assert on WOFF2 tests

https://bugs.webkit.org/show_bug.cgi?id=270414

Reviewed by Michael Catanzaro.

The convertWOFFToSfntIfNecessary() function can set the buffer to nullptr, specially if the
woff2::ConvertWOFF2ToTTF() call in convertWOFFToSfnt() fails. This patch re-introduces the buffer
check that was removed in 274689@main.

* Source/WebCore/loader/cache/CachedFont.cpp:
(WebCore::CachedFont::createCustomFontData):

Canonical link: https://commits.webkit.org/275617@main

Source/WebCore/loader/cache/CachedFont.cpp

@@ -170,7 +170,7 @@ RefPtr<FontCustomPlatformData> CachedFont::createCustomFontData(SharedBuffer& by
 {
     RefPtr buffer = { &bytes };
     wrapping = !convertWOFFToSfntIfNecessary(buffer);
-    return FontCustomPlatformData::create(*buffer, itemInCollection);
+    return buffer ? FontCustomPlatformData::create(*buffer, itemInCollection) : nullptr;
 }
 
 RefPtr<FontCustomPlatformData> CachedFont::createCustomFontDataExperimentalParser(SharedBuffer& bytes, const String& itemInCollection, bool& wrapping)