rrdtool plugin: Fix a possible race condition at startup.

rrdtool plugin: Fix a possible race condition at startup.

On a very busy system, if the network plugin was initialized before the
rrdtool plugin, `rrd_cache_insert' may be called before the RRDtool
plugin is initialized. It would then pass `cache == NULL' to
`c_avl_get', resulting in a segmentation fault.