Only access cmsg->cmsg_len when cmsg is initialized.

When neither IP_PKTINFO nor IP_SENDSRCADDR are defined cmsg is not
initialized and leads to random crashes. While there initialize cmsg
to NULL which would have made it easier to find this bug.
Merge pull request #1405 from fobser/addCMsgSrcAddr

Only access cmsg->cmsg_len when cmsg is initialized.