tools/perf/build: Fix non-existent build directory handling

Arnaldo reported that non-existent build directories were not
recognized  properly. The reason is readlink failure causing 'O'
to become empty.

Solve it by passing through the 'O' variable unmodified if
readlink fails.

Reported-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20131009150023.GA10167@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>