Sktm crashes when submitting Jenkins jobs

[spbnick]

Sktm occasionally crashes when submitting Jenkins jobs, like this:

[21214] 2018-09-19 16:38:41,364     INFO   submitted build: None
Traceback (most recent call last):
  File "/home/skt/.local/bin/sktm", line 11, in <module>
    sys.exit(main())
  File "/home/skt/.local/lib/python2.7/site-packages/sktm/executable.py", line 242, in main
    args.func(sw, cfg)
  File "/home/skt/.local/lib/python2.7/site-packages/sktm/executable.py", line 164, in cmd_patchwork
    sw.check_patchwork()
  File "/home/skt/.local/lib/python2.7/site-packages/sktm/__init__.py", line 296, in check_patchwork
    makeopts=self.makeopts),
  File "/home/skt/.local/lib/python2.7/site-packages/sktm/jenkins.py", line 408, in build
    return build.get_number()
AttributeError: 'NoneType' object has no attribute 'get_number'

A few of these tracebacks could be found in our logs.
This results in Jenkins jobs running, but not reaped by skt, and thus rerun on the next sktm invocation.