Fix bug 1 of T39156: Deadlock while undoing paint operations on image

sequences.

This commit should be included in 2.70 release

source/blender/blenkernel/intern/image.c

@@ -3620,6 +3620,7 @@ ImBuf *BKE_image_get_ibuf_with_name(Image *image, const char *name)
 				IMB_refImBuf(ibuf);
 				break;
 			}
+			IMB_moviecacheIter_step(iter);
 		}
 		IMB_moviecacheIter_free(iter);
 	}