tracing: Avoid NULL return from hist_field_name() on truncation

hist_field_name() returns "" everywhere except the fully-qualified VAR_REF/EXPR case, where snprintf() truncation returns NULL early and bypasses the bottom NULL->"" guard. Callers don't expect NULL:
strcat(expr, hist_field_name(field, 0)) at trace_events_hist.c:1758 and the strcmp() in the sort-key match loop at :4804 both deref it.

system and event_name are bounded by MAX_EVENT_NAME_LEN, but the field name on a VAR_REF is kstrdup'd from a histogram variable name parsed out of the trigger string and has no length cap, so a long enough var name in a fully qualified reference can reach the truncation path.

Keep the length check but leave field_name as "" on overflow.

---
 kernel/trace/trace_events_hist.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

index 0dbbf6cca9bc..eb2c2bc8bc3d 100644
@@ -1369,10 +1369,8 @@ static const char *hist_field_name(struct hist_field *field,
 			len = snprintf(full_name, sizeof(full_name), fmt,
 				       field->system, field->event_name,
 				       field->name);
-			if (len >= sizeof(full_name))
-				return NULL;
-
-			field_name = full_name;
+			if (len < sizeof(full_name))
+				field_name = full_name;
 		} else
 			field_name = field->name;
 	} else if (field->flags & HIST_FIELD_FL_TIMESTAMP)

2.53.0