gvariant: Add an assertion to clarify some tuple logic

This should quell a scan-build error about dereferencing `member_info`
when it’s `NULL` at the end of the function, due to having zero
iterations of the `for` loop.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

glib/gvariant-serialiser.c

@@ -983,6 +983,10 @@ gvs_tuple_needed_size (GVariantTypeInfo         *type_info,
 
   offset = 0;
 
+  /* We must go through at least one iteration below. If the tuple had no
+   * children, it would have a fixed size. */
+  g_assert (n_children > 0);
+
   for (i = 0; i < n_children; i++)
     {
       guint alignment;