Fix finalizer crash in OpenSSLSessionImpl.
Fix finalizer crash in OpenSSLSessionImpl.

bug: 25838479

(cherry picked from commit 0f0237f1a00f4944f06c09dfaec802269e879dfe)

Change-Id: Ic98725c5b90af62a16b630676cf36bda0fa7be6e

google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt