CDB: prevent possible nullptr deref.

Change-Id: I111b4620effc45f26f40880875a70d606c924f69
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/debugger/cdb/cdbengine.cpp

@@ -1800,7 +1800,7 @@ void CdbEngine::handleMemory(const CdbExtensionCommandPtr &command)
 {
     QTC_ASSERT(command->cookie.canConvert<MemoryViewCookie>(), return);
     const MemoryViewCookie memViewCookie = qvariant_cast<MemoryViewCookie>(command->cookie);
-    if (command->success) {
+    if (command->success && memViewCookie.agent) {
         const QByteArray data = QByteArray::fromBase64(command->reply);
         if (unsigned(data.size()) == memViewCookie.length)
             memViewCookie.agent->addLazyData(memViewCookie.editorToken,