In session_process_msg() recheck the validity of the rbuf before moving

In session_process_msg() recheck the validity of the rbuf before moving
the remaining data around.

There is an improbable case where a NOTIFICATION is received while also
reaching the MSG_PROCESS_LIMIT. In this case rbuf is NULL when breaking
out of the for loop and hitting this code.

sthen@ is the (un)lucky person to hit that improbable case
OK tb@ sthen@