src/pmlc/gram.y: fix logic error in pmNewContext() call
src/pmlc/gram.y: fix logic error in pmNewContext() call

Previously was assuming pmNewContext() would return 0 on
success ... that's no longer the case.  Check for < 0 as
the error guard.

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