pmrep: fix a dynamic header corner case with missing values
pmrep: fix a dynamic header corner case with missing values

This is hard to reproduce, seem to need pmrep -1 -X "" and
derived metrics using e.g. rate() with no values available,
affects only the first line of report. Will try to figure
out a bit later a proper test archive to cover this case.

Before:

            PID         UID    %usr %syst           %guest      %CPU
00:00:00 017662         996 84942.0     1          pmproxy    201370

            PID         UID    %CPU   CPU          Command p.p.utime
00:00:00 017662         996  101097     1          pmproxy    239340

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