Fix an issue in which the graph is not displayed.

Graph of phpfpm_connections is not displayed.

/var/log/munin/munin-graph.log

    Unable to graph /var/lib/munin/foo/foo-phpfpm_connections-accepted-d.rrd: opening '/var/lib/munin/foo/foo-phpfpm_connections-Connections-g.rrd': No such file or directory

munin-node v2.0.9