perf report: Skip events with null branch stacks

perf report: Skip events with null branch stacks

A non-synthesized event might not have a branch stack if branch stacks
have been synthesized (using itrace options).

An example of that is when Intel PT records sched_switch events for
decoding purposes.  Those sched_switch events do not have branch stacks
even though the Intel PT decoder may be synthesizing other events that
do due to the itrace options.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1443186956-18718-12-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>