No security checking for UnsafeAccess.getInstance() in UnsafeAccessor

UnsafeAccessor (UA) is a bridge to access jdk.internal.misc.Unsafe & sun.misc.Unsafe. Normally, if UA is loaded as a named module, the internal data of UA is protected by JVM and others can only access UA via UA's standard API. The main application can set up `SecurityCheck.AccessLimiter` for UA to limit access to UA. Starting with version 1.4.0 and prior to version 1.7.0, when `SecurityCheck.AccessLimiter` is set up, untrusted code can access UA without limitation, even when UA is loaded as a named module. This issue does not affect those for whom `SecurityCheck.AccessLimiter` is not set up. Version 1.7.0 contains a patch.