common: initialize fields of Buffer:Slice (#18776)
common: initialize fields of Buffer:Slice (#18776)

The default constructor above does not initialize them.

Signed-off-by: Vitaly Buka <30899b7d84d75ec2c2b981c63ebe46fea5c02e99@google.com>
envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,lyft/envoy