contrib/plugins: fix coverity warning in cache

Coverity complains that appends_stats_line can be fed a 0 leading
to the undefined behaviour of a divide by 0.

Fixes: CID 1519044
Fixes: CID 1519047
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-24-alex.bennee@linaro.org>
1 file changed