Fix a crash when a networked entity is parented to a server-only entity

[RaphaelIT7]

Description

In the case that a networked entity was parented to a server-only entity, it would have resulted in a crash.
This happened due to pEnt-> GetNetworkParent () returning the server-only entity, but when pEdict = pParent->edict(); is executed, pEdict would become NULL since it's server-only and on the next iteration it would crash when attempting to run pEdict->GetNetworkable()

This was originally shown to me by @legokidlogan and was fixed in GMod's dev branch after he reported it there, but I think this is something that should also be fixed here.

Example code to test it with (though I couldn't test it on the latest sdk build due to the engine.dll crashing on startup?)

#if _DEBUG
void CC_Transmit_Crash( const CCommand& args )
{
	// Just some random server-only entity we can parent to
	CBaseEntity *fadeent = dynamic_cast< CBaseEntity * >( CreateEntityByName( "env_fade" ) );
	if ( !fadeent )
	{
		Warning( "Failed to create fade entity!\n" );
		return;
	}

	DispatchSpawn( fadeent );

	// We use the world since for our crash the networked entity MUST use FL_EDICT_ALWAYS
	GetWorldEntity()->SetParent( fadeent );
}
static ConCommand transmit_crash("transmit_crash", CC_Transmit_Crash, "", FCVAR_GAMEDLL);
#endif