perf evlist: Make set_maps() more resilient

Make perf_evlist__set_maps() more resilient by allowing for the
possibility that one or another of the maps isn't being changed and
therefore should not be "put".

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Link: http://lkml.kernel.org/r/1441699142-18905-9-git-send-email-adrian.hunter@intel.com


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