src/libpcp/context.c: fix compilation warnings from Mac OS X
src/libpcp/context.c: fix compilation warnings from Mac OS X

Small changes to placate gcc and comment to explain why gcc got it wrong
in one case.

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