perf annotate: Check null of sym pointer before using it

Sym may be NULL, and that will cause perf to crash.

Signed-off-by: Shan Wei <davidshan@tencent.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/4FCD95D3.90209@gmail.com


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