[EME][GStreamer][Thunder] Fix crash with garbage collected CDMInstance

https://bugs.webkit.org/show_bug.cgi?id=294833

Reviewed by Philippe Normand.

There was a crash in certain situations where the CDMInstance got garbage collected, which made it crash as there was no
valid object. Now we just bail out and warn.

* Source/WebCore/platform/graphics/gstreamer/eme/CDMThunder.cpp:
(WebCore::CDMInstanceSessionThunder::requestLicense):

Canonical link: https://commits.webkit.org/296608@main

Author: calvaris

Source/WebCore/platform/graphics/gstreamer/eme/CDMThunder.cpp

@@ -496,7 +496,11 @@ void CDMInstanceSessionThunder::requestLicense(LicenseType licenseType, KeyGroup
 
     // FIXME: UUID or system ID?
     auto instance = cdmInstanceThunder();
-    ASSERT(instance);
+    if (!instance) {
+        GST_WARNING("no instance, bailing out");
+        callback(WTFMove(initDataSharedBuffer), { }, false, Failed);
+        return;
+    }
     m_initData = InitData(instance->keySystem(), WTFMove(initDataSharedBuffer));
 
     GST_TRACE("Going to request a new session id, init data size %zu", m_initData.payload()->size());