Harden packet validation and replay boundary coverage

[bifurcation]

Summary

• tighten SRTP packet length validation and guard packet key lookup when a stream has no session keys
• make the AES-ICM counter limit and setup failure handling more defensive in edge cases
• add focused regression coverage for short packets, replay database boundaries, and rollover counter edge conditions

Testing

• ./configure --with-crypto-library=openssl && make test/srtp_driver && test/srtp_driver -v
• ./configure --with-crypto-library=internal && make clean && make crypto/test/cipher_driver test/replay_driver test/rdbx_driver test/roc_driver
• crypto/test/cipher_driver -v
• test/replay_driver
• test/rdbx_driver -v
• test/roc_driver