perf db-export: Fix thread ref-counting

Thread ref-counting was not done for get_main_thread() meaning that
there was a thread__get() from machine__find_thread() that was not being
paired with thread__put(). Fix that.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1432906425-9911-2-git-send-email-adrian.hunter@intel.com


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