perf symbols: Check return value of filename__read_debuglink()

When dso__read_binary_type_filename() called, it doesn't check the
return value of filename__read_debuglink() so that it'll try to open the
debuglink file even if it doesn't exist.

Also fix return value of the filename__read_debuglink() as it always
return -1 regardless of the result.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Cody P Schafer <cody@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1392859976-32760-3-git-send-email-namhyung@kernel.org


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