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>