dbi plugin: Removed excessive error reporting

Latest libdbi code sets BADIDX error when no more rows is
in 'dbi_result_next_row()' (similar to 'dbi_result_prev_row()'
and to match library documentation).

There is confusing errors in Collectd logs due to this change.
Added a 'dbi_result_has_next_row()' check to solve.

Closes: #2894