perf bench: Fix NULL pointer dereference in "perf bench all"

The for_each_bench() macro must check that the "benchmarks" field of a
collection is not NULL before dereferencing it because the "all"
collection in particular has a NULL "benchmarks" field (signifying that
it has no benchmarks to iterate over).

This fixes this NULL pointer dereference when running "perf bench all":

  [root@ssdandy ~]# perf bench all
  <SNIP>

  # Running mem/memset benchmark...
  # Copying 1MB Bytes ...

         2.453675 GB/Sec
        12.056327 GB/Sec (with prefault)

  Segmentation fault (core dumped)
  [root@ssdandy ~]#

Signed-off-by: Patrick Palka <patrick@parcs.ath.cx>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1394664051-6037-1-git-send-email-patrick@parcs.ath.cx
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>