fix various issues reported by static analysis

.gitlab-ci.yml

classes/api.php

classes/article.php

classes/auth/base.php

classes/db/prefs.php

classes/feeds.php

classes/logger/syslog.php

classes/opml.php

classes/plugin.php

classes/pluginhost.php

classes/pref/filters.php

classes/pref/users.php

classes/rpc.php

include/errorhandler.php

include/functions.php

include/functions2.php

include/rssfuncs.php

include/sanity_check.php

include/sessions.php

update.php