pbx_realtime: Prevent premature extension matching

pbx_realtime: Prevent premature extension matching

The patterns provided by pbx_realtime were checked in the order in
which they were returned from the realtime backend. If there was
overlap between multiple patterns, the first one to correctly match was
chosen even though it may not have been the best match.

We now sort the patterns descending by their length and compare in that
order. There may be cases where this still results in a sub-optimal
match, but this patch should improve the overall behavior.

ASTERISK-18271 #close
Reported by: Charlie Smurthwaite

Change-Id: I56d9ac15810eb1775966b669c3028e32cc7bd809