Fix NoClassDefFoundError when netty-tcnative is unavailable

Motivation:

ProxyHandlerTest fails with NoClassDefFoundError raised by
SslContext.newClientContext().

Modifications:

Fix a missing 'return' statement that makes the switch-case block fall
through unncecessarily

Result:

- ProxyHandlerTest does not fail anymore.
- SslContext.newClientContext() does not raise NoClassDefFoundError
  anymore.