lookup for the label tag fails, return NULL rather than something close
to NULL. This scenario occurs if mbuf header labeling is optional and
a policy requiring labeling is loaded, resulting in some mbufs having
labels and others not. Previously, 0x14 would be returned because the
NULL from m_tag_find() was not treated specially.
Obtained from: TrustedBSD Project
Sponsored by: DARPA, McAfee Research
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