src/dbpmda/src/pmda.c: fix small memleak on error path
src/dbpmda/src/pmda.c: fix small memleak on error path

Found by cppcheck.

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