Fix the following issue:
  Error: RESOURCE_LEAK
  src/spice-util.c:235: alloc_fn: Storage is returned
         from allocation function "whc_new".
  src/spice-util.c:235: var_assign: Assigning: "ctx" =
         storage returned from "whc_new(instance_obj, gobject)".
  src/spice-util.c:237: leaked_storage: Variable "ctx"
         going out of scope leaks the storage it points to.

  235|       WeakHandlerCtx *ctx = whc_new (instance_obj, gobject);
  236|
  237|->     g_return_val_if_fail (G_TYPE_CHECK_INSTANCE (instance), 0);
  238|       g_return_val_if_fail (detailed_signal != NULL, 0);
  239|       g_return_val_if_fail (c_handler != NULL, 0);

Signed-off-by: Uri Lublin <uril@redhat.com>