does not recover from crash in apt's http method

[Julian Andres Klode]

I just noticed that do-release-upgrade did not properly handle a failure of the http method:

Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/disco", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeMain.py", line 238, in main
    if app.run():
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeController.py", line 2033, in run
    return self.fullUpgrade()
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeController.py", line 1949, in fullUpgrade
    if not self.doDistUpgradeFetching():
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeController.py", line 1185, in doDistUpgradeFetching
    self.cache._fetch_archives(self.fetcher, pm)
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 427, in _fetch_archives
    return self._run_fetcher(fetcher)
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 394, in _run_fetcher
    res = fetcher.run()
apt_pkg.Error: E:Method http has died unexpectedly!, E:Sub-process http received a segmentation fault.
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/problem_report.py", line 497, in add_to_existing
    self.write(f)
  File "/usr/lib/python3/dist-packages/problem_report.py", line 450, in write
    block = f.read(1048576)
  File "/usr/lib/python3.6/codecs.py", line 321, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte

Original exception was:
Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/disco", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeMain.py", line 238, in main
    if app.run():
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeController.py", line 2033, in run
    return self.fullUpgrade()
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeController.py", line 1949, in fullUpgrade
    if not self.doDistUpgradeFetching():
  File "/tmp/ubuntu-release-upgrader-1wz4x5y5/DistUpgrade/DistUpgradeController.py", line 1185, in doDistUpgradeFetching
    self.cache._fetch_archives(self.fetcher, pm)
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 427, in _fetch_archives
    return self._run_fetcher(fetcher)
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 394, in _run_fetcher
    res = fetcher.run()
apt_pkg.Error: E:Method http has died unexpectedly!, E:Sub-process http received a segmentation fault.

It now leaves me stuck:

# do-release-upgrade -d
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.