Cherry-pick c0a7c3b. rdar://problem/114003508

    Regression(266594@main) Crash under NetworkSession::unregisterNetworkDataTask()
    https://bugs.webkit.org/show_bug.cgi?id=259910
    <rdar://113530127>

    Reviewed by J Pascoe.

    NetworkDataTask::m_session is a WeakPtr and may thus be null by the time the
    NetworkDataTask runs. Null check it before calling `unregisterNetworkDataTask()`
    on it.

    * Source/WebKit/NetworkProcess/NetworkDataTask.cpp:
    (WebKit::NetworkDataTask::~NetworkDataTask):

    Canonical link: https://commits.webkit.org/266690@main

Identifier: 265870.299@safari-7616.1.27.12-branch

Source/WebKit/NetworkProcess/NetworkDataTask.cpp

@@ -112,7 +112,8 @@ NetworkDataTask::~NetworkDataTask()
     ASSERT(RunLoop::isMain());
     ASSERT(!m_client);
 
-    m_session->unregisterNetworkDataTask(*this);
+    if (m_session)
+        m_session->unregisterNetworkDataTask(*this);
 }
 
 void NetworkDataTask::scheduleFailure(FailureType type)