Currently, if the service mode is not "global", this code assumes that
Replicated is non-nil. This assumption may not be true in the future.
Instead of making the assumption, explicitly check that Replicated is
non-nil before using it.
Similarly, for limits and reservations, enclose methods that read from
Limits and Reservations within checks that those fields are non-nil.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
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