Fix copying SockAddr struct

Fix copying SockAddr struct

Valgrind alerted about accessing uninitialized bytes after commit
4945e4ed4a:

==700242== VALGRINDERROR-BEGIN
==700242== Conditional jump or move depends on uninitialised value(s)
==700242==    at 0x6D8A2A: getnameinfo_unix (ip.c:253)
==700242==    by 0x6D8BD1: pg_getnameinfo_all (ip.c:122)
==700242==    by 0x4B3EB6: BackendInitialize (postmaster.c:4266)
==700242==    by 0x4B684E: BackendStartup (postmaster.c:4114)
==700242==    by 0x4B6986: ServerLoop (postmaster.c:1780)
==700242==    by 0x4B80CA: PostmasterMain (postmaster.c:1478)
==700242==    by 0x3F7424: main (main.c:197)
==700242==  Uninitialised value was created by a stack allocation
==700242==    at 0x4B6934: ServerLoop (postmaster.c:1737)
==700242==
==700242== VALGRINDERROR-END

That was because the SockAddr struct was not copied correctly.

Per buildfarm animal "skink".