socket: Don't assume m0 != NULL in sbappendcontrol_locked()

Some callers (e.g., ktls_decrypt()) violate this assumption and thus
could trigger a NULL pointer dereference in KMSAN kernels.

Reported by: glebius
Fixes: ec45f952a232 ("sockbuf: Add KMSAN checks to sbappend*()")
MFC after: 1 week