When the last output is disconnected, output_disable is called like
usual and evacuates the output to the root->saved_workspaces list. It
then calls root_for_each_container to remove (untrack) the output from
each container's outputs list. However root_for_each_container did not
iterate the saved workspaces, so when the output gets freed the
containers would have a dangling pointer in their outputs list. Upon
reconnect, container_discover_outputs would attempt to use the dangling
pointer, causing a crash.
This makes root_for_each_container check the saved workspaces list,
which fixes the problem.
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