perf metricgroups: Use zfree() to reduce chances of use after free

Do defensive programming by using zfree() to initialize freed pointers
to NULL, so that eventual use after free result in a NULL pointer deref
instead of more subtle behaviour.

This file already used zfree() in other places, so this just plugs some
leftovers.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>