Avoid "invalid memory address or nil pointer dereference" panic

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)
Merge pull request #5143 from kzys/ns-nil

Avoid "invalid memory address or nil pointer dereference" panic