libcontainer.GetNamespace returns nil on FreeBSD because
libcontainer.namespaceList is empty. In this case, Namespaces#Get should
return nil instead of being panic.
Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Upstream-commit: c5226d94fab4e261fe2407262d9b5177326d4062
Component: engine