[gdk-pixbuf] Avoid a compiler warning

[Matthias Clasen <matthiasc src gnome org>]

commit f0badf75bdd031eda19758ca183c2eea3a8d29b8
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Mar 26 13:17:21 2017 +0200

    Avoid a compiler warning

 gdk-pixbuf/pixops/pixops.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/gdk-pixbuf/pixops/pixops.c b/gdk-pixbuf/pixops/pixops.c
index f91f0e3..f6535f1 100644
--- a/gdk-pixbuf/pixops/pixops.c
+++ b/gdk-pixbuf/pixops/pixops.c
@@ -1794,6 +1794,8 @@ need_to_prescale (double           scale_x,
   case PIXOPS_INTERP_NEAREST:
     /* Doesn't need the optimization */
     return FALSE;
+  default:
+    g_assert_not_reached ();
   }
 
   /* Limit the number of filters created by make_filter_table(). */