Fix T54317: overlapping volume render bug after recent changes.

Increasing the samplig dimensions like this is not optimal, I'm looking
into some deeper changes to reuse the random number and change the RR
probabilities, but this should fix the bug for now.

intern/cycles/render/integrator.cpp

@@ -187,7 +187,9 @@ void Integrator::device_update(Device *device, DeviceScene *dscene, Scene *scene
 		max_samples = max(max_samples, volume_samples);
 	}
 
-	uint total_bounces = max_bounce + transparent_max_bounce + 3 +
+	uint total_bounces = max_bounce +
+	                     transparent_max_bounce + 3 +
+	                     VOLUME_BOUNDS_MAX +
 	                     max(BSSRDF_MAX_HITS, BSSRDF_MAX_BOUNCES);
 
 	max_samples *= total_bounces;