shell: Check for invalid outputs passed on when activating

shell: Check for invalid outputs passed on when activating

Connector hot-plugging would generate new wl_output object, which the
client might re-use so rather than blindly trusting the client, make
sure that the output we're getting is really one suitable.

This should avoid getting an incorrect output upon re-plugging in a
connector.

Bug-AGL: SPEC-4705
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I732283fc16841e0e57ddbec5d8bd2333d5028433
(cherry picked from commit 02fcb317161c08bcf02093a74abe18d4a4392335)