perf evlist: Fix missing thread_map__put in propagate_maps()

perf_evlist__propagate_maps() incorrectly assumes evsel->threads is NULL
before reassigning it, but it won't be NULL when perf_evlist__set_maps()
is used to set different (or NULL) maps.  Thus thread_map__put must be
used, which works even if evsel->threads is NULL.

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/1441699142-18905-7-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>