[DO NOT MERGE] Fix heap buffer overflow in clearkey CryptoPlugin::decrypt

Fix destPtr was not pointing to destination raw pointer.

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: Ia1b8f755daaada2f1411abeb3cb5b832a72b3c82
(cherry picked from commit 8464bfa9)