spice-utils: allocate ctx after g_return_val_if_fail

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>

src/spice-util.c

@@ -231,7 +231,6 @@ gulong spice_g_signal_connect_object (gpointer instance,
                                       GConnectFlags connect_flags)
 {
     GObject *instance_obj = G_OBJECT (instance);
-    WeakHandlerCtx *ctx = whc_new (instance_obj, gobject);
 
     g_return_val_if_fail (G_TYPE_CHECK_INSTANCE (instance), 0);
     g_return_val_if_fail (detailed_signal != NULL, 0);
@@ -240,6 +239,7 @@ gulong spice_g_signal_connect_object (gpointer instance,
     g_return_val_if_fail (
                           (connect_flags & ~(G_CONNECT_AFTER|G_CONNECT_SWAPPED)) == 0, 0);
 
+    WeakHandlerCtx *ctx = whc_new (instance_obj, gobject);
     if (connect_flags & G_CONNECT_SWAPPED)
         ctx->closure = g_cclosure_new_object_swap (c_handler, gobject);
     else