Add fail fast checks to prevent null message, see #2800
(cherry picked from commit 5526f9d2a8b01f944ff078ce57f2a03c21649d05)

* Added null check in Envelope.apply
Merge pull request #1157 from akka/wip-2800-no-null-msg-master-patriknw

Add fail fast checks to prevent null message, see #2800