perf tools: Avoid deadlock when map_groups are broken

Attempting to clone map groups onto themselves will deadlock.

It only happens because of other bugs, but the code should protect
itself anyway.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1439994561-27436-2-git-send-email-adrian.hunter@intel.com


[ Use pr_debug() instead of dump_fprintf() ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>