[niepce] Fix debug build

[Hubert Figuière <hub src gnome org>]

commit 6d9b6dc7864637c0a6e619f3f38eb9e297820422
Author: Hubert Figuière <hub figuiere net>
Date:   Mon Apr 5 23:50:58 2021 -0400

    Fix debug build

 src/niepce/modules/darkroom/imagecanvas.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/niepce/modules/darkroom/imagecanvas.cpp b/src/niepce/modules/darkroom/imagecanvas.cpp
index f7b5cd9..2cf07f0 100644
--- a/src/niepce/modules/darkroom/imagecanvas.cpp
+++ b/src/niepce/modules/darkroom/imagecanvas.cpp
@@ -163,7 +163,7 @@ bool ImageCanvas::on_draw(const Cairo::RefPtr<Cairo::Context>& context)
             } else {
                 img_s = _get_error_placeholder();
             }
-            DBG_ASSERT(img_s, "img_s not loaded");
+            DBG_ASSERT(!!img_s, "img_s not loaded");
             img_w = img_s->get_width();
             img_h = img_s->get_height();
         }