Fix heap buffer overflow in clearkey CryptoPlugin::decrypt

Fix destPtr was not pointing to destination raw pointer.

merge from qt-qpr1-dev: go/ag/9941940

bug: 144506242

Test: sts
  ANDROID_BUILD_TOP= ./android-sts/tools/sts-tradefed run sts-engbuild-no-spl-lock -m StsHostTestCases --test android.security.sts.Poc19_12#testPocBug_144506242

Change-Id: I9fb4ad350ec25dd697940fee57748652745a951d