digital: fixed simple warning on ofdm code.
blocks: updated UDP source to handle no network connections by allowing use of loopback device. Also fixed an infrequent problem on shutdown before the thread exited.

Updated QA code to verify ports used will not clash.
Merge branch 'maint' of git.gnuradio.org:gnuradio into maint
Merge branch 'maint'
Merge branch 'master' into next