The q pointer is updated when the mbuf data is moved from m_dat to
m_ext.

m_ext buffer may also be realloc()'ed and moved during m_cat():
q should also be updated in this case.

Reported-by: Aviv Sasson <asasson@paloaltonetworks.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>