tracing: Add check for NULL event field when creating hist field

Smatch flagged create_hist_field() as possibly being able to
dereference a NULL pointer, although the current code exits in all
cases where the event field could be NULL, so it's not actually a
problem.

Still, to prevent future changes to the code from overlooking new
cases, make the NULL pointer check explicit and warn once in that
case.

Link: http://lkml.kernel.org/r/cfbc003f534a3e441b4313272fd412310aba6336.1461610073.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -371,6 +371,9 @@ static struct hist_field *create_hist_field(struct ftrace_event_field *field,
 		goto out;
 	}
 
+	if (WARN_ON_ONCE(!field))
+		goto out;
+
 	if (is_string_field(field)) {
 		flags |= HIST_FIELD_FL_STRING;