tools lib traceevent: Add sanity check to is_timestamp_in_us()

This patch adds a sanity check to is_timestamp_in_us() input parameter
trace_clock. It avoids a potential segfault in this function for the
case trace_clock is NULL.
Reported-by: Slavomir Kaslev <kaslevs@vmware.com>
Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181128145552.68c4f87b@gandalf.local.homeSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/event-parse.c

@@ -5409,7 +5409,7 @@ void tep_event_info(struct trace_seq *s, struct tep_event_format *event,
 
 static bool is_timestamp_in_us(char *trace_clock, bool use_trace_clock)
 {
-	if (!use_trace_clock)
+	if (!trace_clock || !use_trace_clock)
 		return true;
 
 	if (!strcmp(trace_clock, "local") || !strcmp(trace_clock, "global")