MFC r350815:
In m_pulldown(), before trying to prepend bytes to the subsequent mbuf, ensure that the subsequent mbuf contains the remainder of the bytes the caller sought. If this is not the case, fall through to the code which gathers the bytes in a new mbuf. This fixes a bug where m_pulldown() could fail to gather all the desired bytes into consecutive memory.
PR: 238787
Approved by: so (emaste)