core: when deserializing state always use read_line(…, LONG_LINE_MAX, …)
This should be much better than fgets(), as we can read substantially longer lines and overly long lines result in proper errors. Fixes a vulnerability discovered by Jann Horn at Google. CVE-2018-15686 LP: #1796402 https://bugzilla.redhat.com/show_bug.cgi?id=1639071 (cherry picked from commit 8948b341)
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