netgraph/ng_checksum: Fix double free error

m_pullup(9) frees the mbuf(9) chain in the case of an allocation error.
The mbuf chain must not be freed again in this case.

PR: 255874
Submitted by: <lylgood@foxmail.com>
Approved by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30273