libpcp_web discovery: assign new context before calling pmGetArchiveEnd
libpcp_web discovery: assign new context before calling pmGetArchiveEnd

Last change was not quite right - it moved the pmDiscoverNewSource
call after pmGetArchiveEnd, which ment the context wasn't assigned
correctly in the tracking datastructure.

Fix verified with qa/1661 (and others) now passing again.

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