userspace/snmpd: bugfix remove printing extra item at the end of table

At the end of pstats and ppsi tables extra item was printed. 0 or null as an
output from function netsnmp_extract_iterator_context shall be interpreted as
no more elements instead of element 0 as it was so far.

Fixed ppsi and pstats tables.

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>