src/riemann-client.c: Fix a few signed<->unsigned comparsions

A few functions got size_t as argument, and were comparing it against an
int. Fix that, by using a size_t as the loop variable too.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-client.c: Remove a few unused variables

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>