perf tools: Fix a bug of segmentation fault
perf tools: Fix a bug of segmentation fault

Fix the 'segmentation fault' bug of 'perf list --list-cmds', which also
happens in other cases (e.g. record, report ...). This bug happens when
there are no cmds to list at all.

Example:

Before this patch:

  $ perf list --list-cmds
  Segmentation fault
  $

  After this patch:
  $ perf list --list-cmds
  $

As shown above, the result prints nothing rather than a segmentation
fault. The null result means 'perf list' has no cmds to display at this
time.

Signed-off-by: Yunlong Song <8d07c820a02d28850bd4d06e4d0a5cddd9e128ec@huawei.com>
Cc: Paul Mackerras <19a0ba370c443ba08d20b5061586430ab449ee8c@samba.org>
Cc: Peter Zijlstra <645ca7d3a8d3d4f60557176cd361ea8351edc32b@chello.nl>
Cc: Wang Nan <7b589a40e0ecd4816b422e8ed1f74a01c34bf6bb@huawei.com>
Link: http://lkml.kernel.org/r/1423833115-11199-5-git-send-email-8d07c820a02d28850bd4d06e4d0a5cddd9e128ec@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <293abb6b76d7791c0732cc517d38c4b5c734b87f@redhat.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs