Fix segfault in EncryptionOperationsTest.RsaPkcs1Success

This test tries to modify an encrypted message to ensure that the
result can't be decrypted, but if encrypting the messsage fails
first then there's nothing to modify.

Bug: None
Test: Ran against a Strongbox implementation that refuses to
      encrypt the message using Digest::NONE
Signed-off-by: Bill Richardson <wfrichar@google.com>
Change-Id: Ib4d389a47702edd56a4e7d2b334dc89d0c3972a1