perf tools: Use the DWARF unwind info only if loaded

Use the info only if it has been found in the .debug_frame section of
the ELF binary.

Signed-off-by: Jean Pihet <jean.pihet@linaro.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linaro-kernel@lists.linaro.org
Cc: patches@linaro.org
Link: http://lkml.kernel.org/r/1387212194-8028-1-git-send-email-jean.pihet@linaro.org


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