libpcp - pmns traversal fixups
libpcp - pmns traversal fixups

Fixed a couple of zero-sized malloc() instances in the PM_CONTEXT_LOCAL
case expanding the PMNS below a dynamic metric.  Tightening the guards
prevents the pointless malloc()s.

This was causing some qa/957 fallout.

adfernandes/pcp,andyvand/cygpcpfans,wuliming/pcp,prasincs/pcp,aeg-aeg/pcpfans,mbaldessari/pcp,prasincs/pcp,adfernandes/pcp,tjanez/pcp,tjanez/pcp,prasincs/pcp,wuliming/pcp,aeg-aeg/pcpfans,prasincs/pcp,wuliming/pcp,mbaldessari/pcp,edwardt/pcp,prasincs/pcp,edwardt/pcp,edwardt/pcp,tjanez/pcp,aeg-aeg/pcpfans,adfernandes/pcp,aeg-aeg/pcpfans,prasincs/pcp,wuliming/pcp,wuliming/pcp,tjanez/pcp,tjanez/pcp,wuliming/pcp,tjanez/pcp,edwardt/pcp,aeg-aeg/pcpfans,edwardt/pcp,andyvand/cygpcpfans,prasincs/pcp,adfernandes/pcp,edwardt/pcp,tjanez/pcp,edwardt/pcp,andyvand/cygpcpfans,mbaldessari/pcp,adfernandes/pcp,wuliming/pcp,tjanez/pcp,adfernandes/pcp,adfernandes/pcp,mbaldessari/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,edwardt/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,andyvand/cygpcpfans,mbaldessari/pcp,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,mbaldessari/pcp,aeg-aeg/pcpfans,prasincs/pcp