[metrics-bugs] #24012 [Metrics/Onionoo]: Fix NullPointerExceptions from accessing descriptor parts that may be null

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#24012: Fix NullPointerExceptions from accessing descriptor parts that may be null
-----------------------------+------------------------------
 Reporter:  karsten          |          Owner:  metrics-team
     Type:  defect           |         Status:  needs_review
 Priority:  High             |      Milestone:
Component:  Metrics/Onionoo  |        Version:
 Severity:  Normal           |     Resolution:
 Keywords:                   |  Actual Points:
Parent ID:                   |         Points:
 Reviewer:                   |        Sponsor:
-----------------------------+------------------------------
Changes (by karsten):

 * status:  new => needs_review


Comment:

 Here's the server descriptor:

 {{{
 @type server-descriptor 1.0
 router pearl000 35.203.138.1 9001 0 0
 signing-key
 -----BEGIN RSA PUBLIC KEY-----
 MIGJAoGBAKzTaN4tZGv1kiQWBzeuOk+ovr2LtIURlaVC38j6j/fQuYfuAZX/XvV1
 fQr9EVh+T617dh+frt2D0QDuzLUvP3hpgVozW94w+Ib85pUCne03f4rj3QYu5Qtg
 GvzShslZI6vgyy0g2jAOGa4jxT/UYAcKE5dQo8CBKA6Qb0P5Joc1AgMBAAE=
 -----END RSA PUBLIC KEY-----
 fingerprint 6832 5B4B 1E17 7374 B84D 372F 0304 6351 BEE7 FF6A
 onion-key
 -----BEGIN RSA PUBLIC KEY-----
 MIGJAoGBANN/gLTe05kWKPSEyYJeknuxQst+cVsVmZrZgIYNXuhPn+3XnWhEc10r
 ICa82FkB7hBH6REuW0ugGDc2QLwENmDiaBiFW1LDujEFeVlV8o0VSDwrL3VCPsPL
 zC4/zHqR4DmLFXp5V238MKj85Pud04g65piZCIAsy6hiGMDCoGdtAgMBAAE=
 -----END RSA PUBLIC KEY-----
 ntor-onion-key ATSN2Q9KwCeRu35agh/ChjX8MsgM/FGFRDUX6o9Sbmk
 platform Pearl 0fd5756 on Linux
 contact pearl at m15n.org https://github.com/mmcloughlin/pearl
 bandwidth 153600 307200 153600
 published 2017-10-25 18:00:28
 reject *:*
 proto Link=4 LinkAuth=1 Relay=2
 router-signature
 -----BEGIN SIGNATURE-----
 OyY0vQc5n2RYdkrXqfn09HoACJBx7GrBHZMnmNtlX5nJIL9N4eyyPvmxhmuC+A94
 dDE0u/6w3nCABikFFLHcKaBAdmYBdxrzk3imfVjzYZazHWWr/se8HxK1jibP186A
 8K8bdtMih127CGv3mn+g17uXFTbbuylM7r1xf8NpqRs=
 -----END SIGNATURE-----
 }}}

 Here's the consensus entry:

 {{{
 r pearl000 aDJbSx4Xc3S4TTcvAwRjUb7n/2o 97QY9wS38Dw33qiyzDiGOi4Mu/E
 2017-10-25 04:09:33 35.203.138.1 9001 0
 s Running Valid
 pr Link=4 LinkAuth=1 Relay=2
 w Bandwidth=20 Unmeasured=1
 p reject 1-65535
 }}}

 And
 [https://gitweb.torproject.org/user/karsten/onionoo.git/log/?h=task-24012
 my task-24012 branch] contains what I came up with so far. Please review,
 test, improve!

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24012#comment:1>
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