[babl] babl/base: model-gray, avoid potential NaN

[Øyvind Kolås <ok src gnome org>]

commit 640bad3e7d52d112806095e232a3d989f1a0dd1a
Author: Øyvind Kolås <pippin gimp org>
Date:   Thu Sep 14 18:32:09 2017 +0200

    babl/base: model-gray, avoid potential NaN

 babl/base/model-gray.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/babl/base/model-gray.c b/babl/base/model-gray.c
index d2cf060..6a2764e 100644
--- a/babl/base/model-gray.c
+++ b/babl/base/model-gray.c
@@ -499,7 +499,11 @@ gray_gamma_2_2_premultiplied2rgba (Babl *conversion,
       double alpha = ((double *) src)[1];
       double luminance;
 
-      luma      = luma / alpha;
+      if (alpha > BABL_ALPHA_THRESHOLD)
+        luma      = luma / alpha;
+      else
+        luma      = 0.0;
+
       luminance = babl_trc_to_linear (trc, luma);
 
       ((double *) dst)[0] = luminance;