evl trace:fix the occasional NULL pointer references

Trace event *evl_sched_attrs* calls TP_printk("%s") to out print the thread
name get by evl_element_name().

However, evl_element_name() may return NULL sometimes, and TP_printk(%s)
may cause some problems.

This patch will avoid this.
Signed-off-by: lio <carver4lio@163.com>

include/trace/events/evl.h

@@ -177,7 +177,8 @@ DECLARE_EVENT_CLASS(evl_sched_attrs,
 	),
 
 	TP_printk("thread=%s policy=%s param={ %s }",
-		  evl_element_name(&__entry->thread->element),
+		  evl_element_name(&__entry->thread->element)?
+		  evl_element_name(&__entry->thread->element):"{ }",
 		  evl_print_sched_policy(__entry->policy),
 		  evl_trace_sched_attrs(p,
 					(struct evl_sched_attrs *)