apport crashes with FileNotFoundError in is_same_ns

[errors.ubuntu.com bug bridge]

The Ubuntu Error Tracker has been receiving reports about a problem regarding apport. This problem was most recently seen with package version 2.20.8-0ubuntu3, the problem page at https://errors.ubuntu.com/problem/b6b178fe6ed572189dbfe2627876ed9e9d59ace2 contains more details, including versions of packages affected, stacktrace or traceback, and individual crash reports.
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[Daniel van Vugt (vanvugt)]

This is the top apport crasher by more than an order of magnitude. Over 34000 crashes in the past month alone.

[Brian Murray (brian-murray)]

Here's the full traceback:

Traceback (most recent call last):
  File "/usr/share/apport/apport", line 380, in <module>
    if not is_same_ns(host_pid, "pid") and not is_same_ns(host_pid, "mnt"):
  File "/usr/share/apport/apport", line 305, in is_same_ns
    if os.readlink('/proc/%s/ns/%s' % (pid, ns)) == os.readlink('/proc/self/ns/%s' % ns):
FileNotFoundError: [Errno 2] No such file or directory: '/proc/3558/ns/pid'

I've already committed this fix upstream and I linked the fix to another bug so will mark this as a duplicate of that one.