src/rrd_client.c: Fix a potential segmentation fault in "get_path".

src/rrd_client.c: Fix a potential segmentation fault in "get_path".

The "get_path" function is called at least from "rrdc_update" without
checking for a connection first. In that case the "sd_path" pointer may be
NULL, so dereferencing it without checking for NULL is not good behavior.

This patch checks all arguments of the "get_path" function and returns an
error if any pointer is NULL. -- Florian Forster

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@2094 a5681a0c-68f1-0310-ab6d-d61299d08faa