Not checking for missing attributes in GDTFManager::GdtfDmxChannel::GetName()

[pedromagalhae]

Issue is in GDTFManager::GdtfDmxChannel::GetName(), line 5504:
If fLogicalChannels[0]->GetAttribute() returns nullptr, GetName() will fail.

Checking for nullptr prevents the crash, something like:
if(fLogicalChannels.size() > 0 && fLogicalChannels[0]->GetAttribute()) (...)

But maybe you'd like to handle this error differently.

Attached a description.xml file which is missing the ColorAdd_W attribute.

description.xml

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