[server] avoid a possible race condition on _CnxSetPool.close()
The pool could become empty between time to check and time to use.
extend-diff-ignore = "^(__pkginfo__\.py$|doc/_themes/cubicweb/static/(logo-cubicweb-small\.svg|cubicweb\.ico|logo-cubicweb\.svg)$|cubicweb\.spec$|setup\.cfg$|cubicweb/misc/cwfs)"