pmchart: fix SIGSEGV from opening a view when no archive available
pmchart: fix SIGSEGV from opening a view when no archive available

Resolves Fedora BZ #1256708.

adfernandes/pcp,tjanez/pcp,wuliming/pcp,edwardt/pcp,edwardt/pcp,tjanez/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans,wuliming/pcp,tjanez/pcp,adfernandes/pcp,edwardt/pcp,aeg-aeg/pcpfans,wuliming/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,tjanez/pcp,tjanez/pcp,tjanez/pcp,edwardt/pcp,edwardt/pcp,edwardt/pcp,edwardt/pcp,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,tjanez/pcp,wuliming/pcp,adfernandes/pcp,tjanez/pcp,edwardt/pcp