perf script: No tracepoints? Don't call libtraceevent.

The libtraceevent handler (session->tevent) is only initialized when
there are tracepoints in a perf.data event list, so do not call
pevent_set_function_resolve() in those cases, fixing a segfault.
Reported-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-xyynkucl5p4bcs13zi4i4b1f@git.kernel.org
Report-link: http://lkml.kernel.org/r/20150803174113.GA20282@krava.redhat.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-script.c

@@ -1861,7 +1861,8 @@ int cmd_script(int argc, const char **argv, const char *prefix __maybe_unused)
 	else
 		symbol_conf.use_callchain = false;
 
-	if (pevent_set_function_resolver(session->tevent.pevent,
+	if (session->tevent.pevent &&
+	    pevent_set_function_resolver(session->tevent.pevent,
 					 machine__resolve_kernel_addr,
 					 &session->machines.host) < 0) {
 		pr_err("%s: failed to set libtraceevent function resolver\n", __func__);