src/libpcp/src/pmns.c: fix malloc corruption for PMNS load/unload operations
src/libpcp/src/pmns.c: fix malloc corruption for PMNS load/unload operations

Only dbpmda is really able to exercise this, but if you load a PMNS,
unload it, load another one (and especially if the second load fails
for some reason) the internal state of the PMNS was bad, leading to
all sorts of corruption at a subsequent load or unload.

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