libpcp_web, qa: fix recent regression in test 662
libpcp_web, qa: fix recent regression in test 662

Fixed regression picked up by test 662 relating to pmStore(3)
interacting incorrectly with following pmFetch(3) on metrics
with instances.  Fallout from commit c88e1c906780.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp