Fix Heap buffer overflow in clearkey releaseSecureStops

Security Vulnerability fix: Heap buffer overflow in clearkey
releaseSecureStops - android.hardware.drm@1.1-service.clearkey

Fix provided by researcher.

Test: CTS tests
  android.media.cts.MediaDrmClearkeyTest#testSecureStop
  android.media.cts.MediaDrmClearkeyTest

Test: run drmpoc
  no signal 6 on clearkey service after the fix

Test: adb shell ps | grep clearkey
  pid ID does not change after running drmpoc

bug: 137284652
Change-Id: Id085945943aa6b85c597dc8b7c32a3b990c54a2b