Make the AS_SET handling for source-as even simpler. It is enough to only

Make the AS_SET handling for source-as even simpler. It is enough to only
extract the rightmost AS of a segment if the segment is not an AS_SET.
Then if we hit the final segment as will contain the last aggregator AS.
This fixes a possible issue with a path like 1 2 3 { 4 5 } { 6 7 } which
should match for source-as 3.
OK benno@