pkg: Fix Coverity warnings

[markj]

Summary

• Fix allocation size in config_get_repositories().
• Fix a memory leak in read_conf_file().
• Avoid a null pointer dereference in an error path in verify_pubsignature().

Fixes: e3b4a51580fc ("pkg(7): expand VERSION_MAJOR, VERSION_MINOR, RELEASE and OSNAME")
Fixes: dc4581589a32 ("pkg: clean support for repositories")

[markj]

Committed in https://cgit.freebsd.org/src/commit/?id=c1557708f1fae1bb9c8e23e3bbb2aa2b055e1211, I forgot to tag the review.