iface-modem-3gpp,ussd: let send_finish() return a newly allocated string

GTask integration will benefit from this change.

src/mm-broadband-modem.c

@@ -4970,10 +4970,7 @@ modem_3gpp_ussd_send_finish (MMIfaceModem3gppUssd *self,
     if (g_simple_async_result_propagate_error (G_SIMPLE_ASYNC_RESULT (res), error))
         return NULL;
 
-    /* We can return the string as constant because it is owned by the async
-     * result, which will be valid during the whole call of its callback, which
-     * is when we're actually calling finish() */
-    return (const gchar *)g_simple_async_result_get_op_res_gpointer (G_SIMPLE_ASYNC_RESULT (res));
+    return g_strdup (g_simple_async_result_get_op_res_gpointer (G_SIMPLE_ASYNC_RESULT (res)));
 }
 
 static void modem_3gpp_ussd_context_step (Modem3gppUssdSendContext *ctx);

src/mm-iface-modem-3gpp-ussd.c

@@ -194,15 +194,17 @@ handle_respond_ready (MMIfaceModem3gppUssd *self,
                       HandleRespondContext *ctx)
 {
     GError *error = NULL;
-    const gchar *reply;
+    gchar *reply;
 
-    reply = MM_IFACE_MODEM_3GPP_USSD_GET_INTERFACE (self)->send_finish (self, res,&error);
+    reply = MM_IFACE_MODEM_3GPP_USSD_GET_INTERFACE (self)->send_finish (self, res, &error);
     if (!reply)
         g_dbus_method_invocation_take_error (ctx->invocation, error);
-    else
+    else {
         mm_gdbus_modem3gpp_ussd_complete_respond (ctx->skeleton,
                                                   ctx->invocation,
                                                   reply);
+        g_free (reply);
+    }
     handle_respond_context_free (ctx);
 }
 
@@ -298,15 +300,17 @@ handle_initiate_ready (MMIfaceModem3gppUssd *self,
                        HandleInitiateContext *ctx)
 {
     GError *error = NULL;
-    const gchar *reply;
+    gchar *reply;
 
     reply = MM_IFACE_MODEM_3GPP_USSD_GET_INTERFACE (self)->send_finish (self, res, &error);
     if (!reply)
         g_dbus_method_invocation_take_error (ctx->invocation, error);
-    else
+    else {
         mm_gdbus_modem3gpp_ussd_complete_initiate (ctx->skeleton,
                                                    ctx->invocation,
                                                    reply);
+        g_free (reply);
+    }
     handle_initiate_context_free (ctx);
 }
 

src/mm-iface-modem-3gpp-ussd.h

@@ -93,9 +93,9 @@ struct _MMIfaceModem3gppUssd {
                    const gchar *command,
                    GAsyncReadyCallback callback,
                    gpointer user_data);
-    const gchar * (* send_finish) (MMIfaceModem3gppUssd *self,
-                                   GAsyncResult *res,
-                                   GError **error);
+    gchar * (* send_finish) (MMIfaceModem3gppUssd *self,
+                             GAsyncResult *res,
+                             GError **error);
 
     /* Cancel */
     void (* cancel) (MMIfaceModem3gppUssd *self,