libpcp_pmda - pmdaTreeChildren tweak for dynamic metrics
libpcp_pmda - pmdaTreeChildren tweak for dynamic metrics

Tighten the guard to prevent a couple of malloc(0) cases that
were causing qa/957 fallout.

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