Cycles: Solve some harmless NULL pointer magic

Was harmless but confused some sanity checks, also kinda makes sense
to be more verbose about what's going on there.

intern/cycles/device/device_memory.h

@@ -212,11 +212,14 @@ public:
 	{
 		data_size = width * ((height == 0)? 1: height) * ((depth == 0)? 1: depth);
 		data.resize(data_size);
-		data_pointer = (device_ptr)&data[0];
 		data_width = width;
 		data_height = height;
 		data_depth = depth;
-
+		if(data_size == 0) {
+			data_pointer = 0;
+			return NULL;
+		}
+		data_pointer = (device_ptr)&data[0];
 		return &data[0];
 	}