[MSE] CompletionHandler may not always be called in SourceBufferPrivate::didReceiveInitializationSegment

[Jean-Yves Avenard [:jya]]

If the initialisation segment received in SourceBufferPrivate::didReceiveInitializationSegment is invalid; then the CompletionHandler will not be called; which would assert in the CompletionHandler destructor
void SourceBufferPrivate::didReceiveInitializationSegment(SourceBufferPrivateClient::InitializationSegment&& segment, CompletionHandler<void()>&& completionHandler)
{
    if (!m_client) {
        completionHandler();
        return;
    }

    if (m_receivedFirstInitializationSegment && !validateInitializationSegment(segment)) {
        m_client->sourceBufferPrivateAppendError(true);
        return;
    }
...
}


if validateInitializationSegment(segment) returns false; then we will error, and return. But this would assert once completionHandler goes out of scope.

[Radar WebKit Bug Importer]

<rdar://problem/80446981>