libpcp: remove an impossible-to-reach guard
libpcp: remove an impossible-to-reach guard

Ken found compiler warnings on Mac OS X due to the
type of a field in the label data structure being
unsigned, but code had checks for less-than-zero;
drop these as they cannot ever be reached.

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