[gimp] app: fix NULL mask handling in previous commit

[N/A <ell src gnome org>]

commit 8e68ca855ddf7716bda8c59fc925205934d05e5e
Author: Ell <ell_se yahoo com>
Date:   Wed Mar 8 15:28:26 2017 -0500

    app: fix NULL mask handling in previous commit

 .../layer-modes/gimpoperationlayermode.c           |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/app/operations/layer-modes/gimpoperationlayermode.c 
b/app/operations/layer-modes/gimpoperationlayermode.c
index ef8a5bb..683ab59 100644
--- a/app/operations/layer-modes/gimpoperationlayermode.c
+++ b/app/operations/layer-modes/gimpoperationlayermode.c
@@ -1042,7 +1042,8 @@ gimp_composite_blend (GimpOperationLayerMode *layer_mode,
 
       in      += 4 * GIMP_COMPOSITE_BLEND_MAX_SAMPLES;
       layer   += 4 * GIMP_COMPOSITE_BLEND_MAX_SAMPLES;
-      mask    +=     GIMP_COMPOSITE_BLEND_MAX_SAMPLES;
+      if (mask)
+        mask  +=     GIMP_COMPOSITE_BLEND_MAX_SAMPLES;
       out     += 4 * GIMP_COMPOSITE_BLEND_MAX_SAMPLES;
 
       samples -= GIMP_COMPOSITE_BLEND_MAX_SAMPLES;