coding style: avoid copying IPv6 source/dest addresses at once
coding style: avoid copying IPv6 source/dest addresses at once

Coverity Scan detected that the TCP profile was copying both IPv6 source
and destination addresses at once. While it is not a bug since the
struct are packed, performing 2 copies instead of 1 improves the code
readability and the overall coding style.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc