Fix mbuf releated crashes in switch(4). They have been found by

Fix mbuf releated crashes in switch(4).  They have been found by
syzkaller as pool corruption panic.  It is unclear which bug caused
what, but it should be better now.
- Check M_PKTHDR with assertion before accessing m_pkthdr.
- Do not access oh_length without m_pullup().
- After checking if there is space at the end of the mbuf, don't
  overwrite the data at the beginning.  Append the new content.
- Do not set m_len and m_pkthdr.len when it is unclear whether
  the ofp_error header fits at all.  Use m_makespace() to adjust
  the mbuf.
Reported-by: syzbot+6efc0a9d5b700b54392e@syzkaller.appspotmail.com
test akoshibe@; OK claudio@