ftrace: Test for NULL iter->tr in regex for stack_trace_filter changes

As writing into stack_trace_filter, the iter-tr is not set and is NULL.
Check if it is NULL before dereferencing it in ftrace_regex_release().

Fixes: 8c08f0d5 ("ftrace: Have cached module filters be an active filter")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -5043,7 +5043,7 @@ int ftrace_regex_release(struct inode *inode, struct file *file)
 
 		if (filter_hash) {
 			orig_hash = &iter->ops->func_hash->filter_hash;
-			if (!list_empty(&iter->tr->mod_trace))
+			if (iter->tr && !list_empty(&iter->tr->mod_trace))
 				iter->hash->flags |= FTRACE_HASH_FL_MOD;
 		} else
 			orig_hash = &iter->ops->func_hash->notrace_hash;