libpcp: remove impossible to reach conditional (coverity)
libpcp: remove impossible to reach conditional (coverity)

pmGetArchiveEnd_ctx cannot take a null ctxp pointer as
it dereferences it immediately (which would segv) - so
make the context-locked check unconditional.

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