stats: Fix name prefix handling

Currently the having an unset prefix leads to an abort() in the
statsd reporter due to an fprintf format string error. In addition
the prefix cannot be reset to its initial state (NULL) by using 'no
prefix', which just sets the prefix to the empty string, causing a
single leading dot to appear in front of the name.

This commit changes the implemenation to consistly use NULL for
the unset name prefix ('no prefix') and to handle this case correctly
in the statsd reporter.

Sponsored-by: On-Waves ehf
1 file changed