rml/oob: check peer param in send methods

[hppritcha]

The rml/oob was not doing sanity checks on the input peer
parameter for the orte_rml_oob_send_nb and orte_rml_oob_send_buffer_nd.
Owing to the fact that there are places in the ompi/orte stack
where things like orte_show_help_norender are called way before
ORTE_PROC_MY_HNP, etc. are setup properly, all kinds of weird
startup failures can occur as the rml/oob tries to process send
requests where the peer is junk.

Rather than try to expand this kind of thing:

/* if we are the HNP, or the RML has not yet been setup,
 * or ROUTED has not been setup,
 * or we weren't given an HNP, or we are running in standalone
 * mode, then all we can do is process this locally
 */
if (ORTE_PROC_IS_HNP || orte_standalone_operation ||
    NULL == orte_rml.send_buffer_nb ||
    NULL == orte_routed.get_route ||
    NULL == orte_process_info.my_hnp_uri) {
    rc = show_help(filename, topic, output, ORTE_PROC_MY_NAME);
}

do the right thing in the rml level and return an error rather than
eventually failing in the send owing to peer not being valid.

@jsquyres please review

[mellanox-github]

Refer to this link for build results (access rights to CI server needed):
http://bgate.mellanox.com/jenkins/job/gh-ompi-master-pr/185/
Test PASSed.