Another out-of-bounds error caught by randomizing memory.

src/mame/drivers/cubeqst.c

@@ -128,6 +128,8 @@ static VIDEO_UPDATE( cubeqst )
 		memset(depth_buffer, 0xff, 512);
 
 		/* Process all the spans on this scanline */
+		if (y < 256)
+		{
 			for (i = 0; i < num_entries; i += 2)
 			{
 				int color = 0, depth = 0;
@@ -171,6 +173,7 @@ static VIDEO_UPDATE( cubeqst )
 				}
 			}
 		}
+	}
 
 	return 0;
 }