tracing: Add NULL trace-array check in print_synth_event()

Add NULL trace-array check in print_synth_event(), because
if we enable tp_printk option, iter->tr can be NULL.

Link: http://lkml.kernel.org/r/157867236536.17873.12529350542460184019.stgit@devnote2Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -833,7 +833,7 @@ static enum print_line_t print_synth_event(struct trace_iterator *iter,
 		fmt = synth_field_fmt(se->fields[i]->type);
 
 		/* parameter types */
-		if (tr->trace_flags & TRACE_ITER_VERBOSE)
+		if (tr && tr->trace_flags & TRACE_ITER_VERBOSE)
 			trace_seq_printf(s, "%s ", fmt);
 
 		snprintf(print_fmt, sizeof(print_fmt), "%%s=%s%%s", fmt);