css: Avoid a crash while parsing radial gradients

has_size does not imply that sizes[0] is != NULL.

(cherry picked from commit aceba7484f)

gtk/gtkcssimageradial.c

@@ -346,7 +346,7 @@ gtk_css_image_radial_parse (GtkCssImage  *image,
 
   if (has_size && !radial->circle)
     {
-      if (!radial->sizes[1])
+      if (radial->sizes[0] && !radial->sizes[1])
         radial->sizes[1] = _gtk_css_value_ref (radial->sizes[0]);
     }