Fix wrap-around bug when displaying IP info (-y option)

Fix wrap-around bug when displaying IP info (-y option)

Some IP prefixes have an allocation date, but some don't.
For instance:

$ dig +short 49.60.231.80.origin.asn.cymru.com TXT
"6453 | 80.231.0.0/16 | EU | ripencc | 2002-03-20"

$ dig +short 186.83.219.195.origin.asn.cymru.com TXT
"6453 | 195.219.0.0/16 | EU | ripencc |"

In the latter case, the parsing code would miss the last (empty) field
and wrap around to the AS number, producing an inconsistent output
(see below).

While we're at it, also make the parsing logic easier to understand...

Before this patch:

$ mtr --report -y 4 nic.co.jp
Start: Thu Jun  9 19:13:50 2016
HOST: gilead                      Loss%   Snt   Last   Avg  Best  Wrst StDev
  1. 2004-04-05 gatwick.net.cl.ca  0.0%    10    2.0   4.7   2.0   7.1   1.9
  2. 786        cl-wgb.route-west  0.0%    10    0.6  11.3   0.6 104.7  32.8
  3. 786        d-we.c-ce.net.cam  0.0%    10    0.7   0.6   0.5   0.8   0.0
  4. 786        c-ce.b-ec.net.cam  0.0%    10    0.7   6.9   0.3  52.6  16.2
  5. 786        ae1.cambab-rbr1.e  0.0%    10    0.3   0.5   0.3   0.7   0.0
  6. 786        146.97.65.118      0.0%    10    0.7   0.6   0.5   0.8   0.0
  7. 786        146.97.38.17       0.0%    10    3.5   9.3   3.5  17.9   6.2
  8. 786        ae29.londtn-sbr1.  0.0%    10    3.7   4.0   3.5   6.6   0.9
  9. 2002-03-20 ix-ae-11-0.tcore1  0.0%    10    3.8   5.0   3.5  16.8   4.1
 10. 6453       195.219.83.186     0.0%    10    4.3   4.4   3.9   5.6   0.3
 11. 2914       ae-4.r22.londen03  0.0%    10    7.7   8.8   3.9  25.5   6.9
 12. 2914       ae-6.r21.sngpsi05  0.0%    10  185.4 185.8 174.4 226.5  14.9
 13. 2914       ae-20.r31.tokyjp0  0.0%    10  249.9 250.0 249.8 250.4   0.0
 14. 2914       ae-21.r00.tokyjp0  0.0%    10  265.9 265.9 265.6 266.1   0.0
 15. 2000-09-29 ae-0.gmo.tokyjp05  0.0%    10  253.7 252.5 244.7 257.5   4.4
 16. ???        ???               100.0    10    0.0   0.0   0.0   0.0   0.0
 17. 7506       g-o-p-4ee-a01-1-e  0.0%    10  250.3 250.3 250.2 250.5   0.0
 18. 7506       unused-157-7-041-  0.0%    10  257.0 256.9 256.7 257.2   0.0
 19. 7506       157-7-107-115.vir  0.0%    10  258.6 258.4 258.2 258.6   0.0

After this patch:

$ mtr --report -y 4 nic.co.jp
Start: Thu Jun  9 19:12:30 2016
HOST: gilead                      Loss%   Snt   Last   Avg  Best  Wrst StDev
  1. 2004-04-05 gatwick.net.cl.ca  0.0%    10    3.6   7.3   1.2  18.1   5.9
  2.            cl-wgb.route-west  0.0%    10    0.6   0.8   0.5   1.3   0.0
  3.            d-we.c-ce.net.cam  0.0%    10    0.5   0.7   0.5   1.0   0.0
  4.            c-ce.b-ec.net.cam  0.0%    10    0.4   1.6   0.3   9.1   2.6
  5.            ae1.cambab-rbr1.e  0.0%    10    0.4   0.5   0.3   0.7   0.0
  6.            146.97.65.118      0.0%    10    0.5   1.1   0.5   5.0   1.3
  7.            146.97.38.17       0.0%    10    3.7   8.4   3.4  51.9  15.3
  8.            ae29.londtn-sbr1.  0.0%    10    3.7   9.0   3.5  54.8  16.1
  9. 2002-03-20 ix-ae-11-0.tcore1  0.0%    10    5.5   4.1   3.5   6.0   0.8
 10.            195.219.83.186     0.0%    10    4.0   4.2   3.9   4.6   0.0
 11.            ae-4.r22.londen03  0.0%    10    3.8   9.5   3.8  33.6  11.6
 12.            ae-6.r21.sngpsi05 20.0%    10  185.8 181.0 179.5 185.8   2.1
 13.            ae-20.r31.tokyjp0  0.0%    10  249.9 249.7 249.5 249.9   0.0
 14.            ae-21.r00.tokyjp0  0.0%    10  266.1 266.0 265.7 266.4   0.0
 15. 2000-09-29 ae-0.gmo.tokyjp05  0.0%    10  243.8 243.8 243.4 244.4   0.0
 16. ???        ???               100.0    10    0.0   0.0   0.0   0.0   0.0
 17.            g-o-p-4ee-a01-1-e  0.0%    10  246.1 246.2 245.7 247.4   0.0
 18.            unused-157-7-041-  0.0%    10  257.3 258.0 256.9 260.9   1.2
 19.            157-7-107-115.vir  0.0%    10  258.3 258.4 258.2 258.8   0.0