app: fix rendering background of Colormap dockable.

The rgba variable would be used uninitialized. This needs to be moved
one loop level up so that the color extracted at each cell start
position is properly reused (and also so that the full transparent color
at no-entry position is properly set and reused too).

app/widgets/gimpviewrendererpalette.c

@@ -161,13 +161,12 @@ gimp_view_renderer_palette_render (GimpViewRenderer *renderer,
 
       if ((y % renderpal->cell_height) == 0)
         {
+          guchar  rgba[4];
           gint    n = 0;
           guchar *d = row;
 
           for (x = 0; x < renderer->width; x++, d += 4)
             {
-              guchar rgba[4];
-
               if ((x % renderpal->cell_width) == 0)
                 {
                   if (list && n < renderpal->columns &&