libgimpwidgets: handle invalid enum values in gimp_enum_store_set_icon_prefix()

If the store has invalid enum values the function segfaults,
instead of just ignoring them.  We use -1 to represent separator
rows in the layer mode combo; if we end up adding icons to this
combo, we need it... not to segfault.  Since this is public API,
it should probably be prepared to handle such uses regardless.

libgimpwidgets/gimpenumstore.c

@@ -375,9 +375,12 @@ gimp_enum_store_set_icon_prefix (GimpEnumStore *store,
 
           enum_value = g_enum_get_value (store->enum_class, value);
 
-          icon_name = g_strconcat (icon_prefix, "-",
-                                   enum_value->value_nick,
-                                   NULL);
+          if (enum_value)
+            {
+              icon_name = g_strconcat (icon_prefix, "-",
+                                       enum_value->value_nick,
+                                       NULL);
+            }
         }
 
       gtk_list_store_set (GTK_LIST_STORE (store), &iter,