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@devnote2



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>