[VPP-1728] Error unformatting FIB paths

[vvalderrv]

CLI:
abf policy add id 9001 acl 0 via ip4-lookup-in-table 42

generates:
2019 Jul 28 10:30:08 received signal SIGSEGV, PC 0x7fc588126c6c, faulting address 0xffffffffffffffff
2019 Jul 28 10:30:08 #0 0x00007fc5875b604b 0x7fc5875b604b
2019 Jul 28 10:30:08 #1 0x00007fc586edc890 0x7fc586edc890
2019 Jul 28 10:30:08 #2 0x00007fc588126c6c unformat_fib_route_path + 0x3ec
2019 Jul 28 10:30:08 #3 0x00007fc586a9e5bb va_unformat + 0x6bb
2019 Jul 28 10:30:08 #4 0x00007fc586a9ec67 unformat + 0x87
2019 Jul 28 10:30:08 #5 0x00007fc545aa1114 0x7fc545aa1114
2019 Jul 28 10:30:08 #6 0x00007fc58754d859 0x7fc58754d859
2019 Jul 28 10:30:08 #7 0x00007fc58754db95 0x7fc58754db95
2019 Jul 28 10:30:08 #8 0x00007fc58754df44 vlib_cli_input + 0xa4
2019 Jul 28 10:30:08 #9 0x00007fc5875afbea 0x7fc5875afbea
2019 Jul 28 10:30:08 #10 0x00007fc5875b3998 0x7fc5875b3998
2019 Jul 28 10:30:08 #11 0x00007fc58756ae36 0x7fc58756ae36
2019 Jul 28 10:30:08 #12 0x00007fc586a597fc 0x7fc586a597fc