path: Update Join (minor optimization)
path: Update Join (minor optimization)

Benchmark comparison:

benchmark                       old ns/op     new ns/op     delta
BenchmarkJoin/10_Paths-8        24.9          23.1          -7.23%
BenchmarkJoin/100_Paths-8       148           141           -4.73%
BenchmarkJoin/1000_Paths-8      1121          1065          -5.00%
BenchmarkJoin/10000_Paths-8     14071         13642         -3.05%

Change-Id: If26df888262f3117579a7552d9b0225c090c7926

aristanetworks/goarista,aristanetworks/goarista