mc_compact: Record-Route is now mandatory. Also Content-Type is mandatory only if Content-Length > 0
tm: check if the reply transaction is defined

closes #388
ds_is_in_list: properly pass -1 for all sets

Related to #389
dispatcher: fix several issues with empty destination sets

Upon (re)load, a dispatching set may be empty if none of its destinations
can be resolved. This patch fixes a couple of crashes which are directly
related to empty destination sets:

    * fix a bug in reindex_dests() where a single bogus destination
      would be allocated for each empty set
    * prevent ds_select_dst() from routing to empty sets
rtpproxy: properly handle ipv6 IPs

Close #394
mc_compact: Contact header is mandatory
Merge remote-tracking branch 'upstream/master'