message: Don't try to access i915-perf data if no file was given
  With USE_I915_PERF=1, if the user opens a .dat file without a
  .i915-dat one, we run into a segfault... Oopsie
diff: @@ -98,6 +98,9 @@ void I915PerfCounters::init( TraceEvents &trace_events )
  m_counters.clear();
  + if (!m_trace_events->i915_perf_reader)
  + return;
  +
  const struct intel_perf_metric_set *metric_set =
  m_trace_events->i915_perf_reader->metric_set;
  