src/libpcp/src/derive_fetch.c: fix ternary expression corner case
src/libpcp/src/derive_fetch.c: fix ternary expression corner case

We require at least one value for each of the guard, the true expression
and the false expression ... the guard test was missing.

Fixes the problem exposed by qa/1203.

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