The clear intent here is to do m_pullup() if we _don't_ have two bytes.
Before the fix: a) m_pullup() was always called and did nothing; b) if a
node was ever supplied with a fragmented but otherwise correct mbuf, then
it would not handle PROT_LCP as intended.
Noticed by: zlei
Fixes: 28903f396af4b151e16ea606cda66a9244fb179f