Do not throw exception on Mac#reset

OpenSSLMac was not checking whether it was initialized before
dereferencing its macKey field. This caused callers to Mac#reset()
before Mac#init() to get a NullPointerException.

Bug: https://code.google.com/p/android/issues/detail?id=58179
Change-Id: I8523983fec578079a66a04d585c1ddbe7732575e