[EME][Thunder] Avoid crashing with null answers during session load

https://bugs.webkit.org/show_bug.cgi?id=296690

Reviewed by Philippe Normand.

OpenCDM/Thunder can answer with a null message when loading a wrong session. We need to deal with that.

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

Canonical link: https://commits.webkit.org/298032@main

Author: calvaris

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

@@ -624,7 +624,7 @@ void CDMInstanceSessionThunder::loadSession(LicenseType, const String& sessionID
             }
         } else {
             GST_ERROR("session %s not loaded", m_sessionID.utf8().data());
-            if (responseMessage->isEmpty())
+            if (!responseMessage || responseMessage->isEmpty())
                 callback(std::nullopt, std::nullopt, std::nullopt, SuccessValue::Failed, sessionLoadFailureFromThunder({ }));
             else {
                 auto responseData = responseMessage->extractData();
@@ -634,8 +634,10 @@ void CDMInstanceSessionThunder::loadSession(LicenseType, const String& sessionID
             }
         }
     });
-    if (!m_session || m_sessionID.isEmpty() || opencdm_session_load(m_session->get()))
+    if (!m_session || m_sessionID.isEmpty() || opencdm_session_load(m_session->get())) {
+        GST_DEBUG("loading failed");
         sessionFailure();
+    }
 }
 
 void CDMInstanceSessionThunder::closeSession(const String& sessionID, CloseSessionCallback&& callback)