SockDiag - fix a deadcode warning

Assignments in if statements are ugly anyway...

Fixes:
  system/netd/server/SockDiag.cpp:146:10: error: Although the value stored to 'ret' is used in the enclosing expression, the value is never actually read from 'ret' [clang-analyzer-deadcode.DeadStores,-warnings-as-errors]
    if ((ret = getaddrinfo(addrstr, nullptr, &hints, &res)) != 0) {
         ^

Test: builds
Bug: 155351989
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ia5ad0ef3176b90915cba6c8a1b8626ead53d67f6