Add defensive lifetime management for security metadata (#152)

Motivation:

Right now we're playing a little fast and loose with the lifetimes of
the sec_protocol_metadata_t. As a practical matter it is highly likely
that this is owned (and so kept alive by) the NWConnection, but rather
than risk that we should tighten up the lifetime management.

Modifications:

Use withExtendedLifetime to extend the lifetime.

Result:

Better lifetime management.

Sources/NIOTransportServices/NIOTSConnectionChannel.swift

@@ -795,9 +795,16 @@ extension NIOTSConnectionChannel {
 
         if let metadata = self.nwConnection?.metadata(definition: NWProtocolTLS.definition) as? NWProtocolTLS.Metadata {
             // This is a TLS connection, we may need to fire some other events.
-            let negotiatedProtocol = sec_protocol_metadata_get_negotiated_protocol(metadata.securityProtocolMetadata).map {
-                String(cString: $0)
+            let securityMetadata = metadata.securityProtocolMetadata
+
+            // The pointer returned by `sec_protocol_metadata_get_negotiated_protocol` is presumably owned by it, so we need
+            // to confirm it's still alive while we copy the data out.
+            let negotiatedProtocol = withExtendedLifetime(securityMetadata) {
+                sec_protocol_metadata_get_negotiated_protocol(metadata.securityProtocolMetadata).map {
+                    String(cString: $0)
+                }
             }
+
             self.pipeline.fireUserInboundEventTriggered(TLSUserEvent.handshakeCompleted(negotiatedProtocol: negotiatedProtocol))
         }
     }