src/libpcp/src/derive_fetch.c: bin_op() fix for Coverity
src/libpcp/src/derive_fetch.c: bin_op() fix for Coverity

Just being a bit more anal in the "op" switches and making the
"should not happen" default case explicit rather than implicit.

Fixes: CID 366084

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