lib/linux_ns.c: fix some possible null pointer dereferences

In case we cannot open /self/proc/mounts or it is malformed.
Spotted by Coverity, CID #971002, #971003.
plugins/carbon: add missing call to va_end()

Reported by Coverity as CID #971091.
core: fix copy & paste bug in uwsgi_manage_command_cron

It looks like tm_hour is used where tm_mon was intended.
Reported by Coverity as CID #970975.
core: add missing unlock in wsgi_req_accept

Reported by Coverity as CID #970989.
core: add missing legion mutex unlock on error path

Reported by Coverity as CID #970988.
Merge pull request #132 from xrmx/coverity2a

More issues reported by Coverity