backend-rdp: Correctly tear down the RDP back-end
This fixes the tear-down and the destroying part in case RDP back-end couldn't be initialized. The first issue is the rdp_output which will not be created in some circumstances (can't open the socket for instance) and requires a guard check, and secondly, the rdp_head being created above of that, wasn't removed and tripped an assert when destroying the compositor instance.
Signed-off-by: Marius Vlad marius.vlad@collabora.com