m_getptr(m, 0, ...) may return an mbuf different from m -- if m has no

m_getptr(m, 0, ...) may return an mbuf different from m -- if m has no
data in it. m_getptr() hops over empty buffers and points to the first
allocated data byte. Because of this the m_dup_pkthdr() call done by
m_copym0() can panic because not the first mbuf is passed.
Found the hard way by myself, diff by blambert@ commiitting for him since
he is not around. Tested and OK myself