lib:async_req: Initialize variables

lib:async_req: Initialize variables

../../lib/async_req/async_sock.c: In function ‘writev_do’:
../../lib/async_req/async_sock.c:360:12: error: ‘written’ may be used uninitialized [-Werror=maybe-uninitialized]
  360 |         if ((written == -1) &&
      |            ^
../../lib/async_req/async_sock.c:343:17: note: ‘written’ was declared here
  343 |         ssize_t written;
      |                 ^~~~~~~
../../lib/async_req/async_sock.c: In function ‘read_packet_do’:
../../lib/async_req/async_sock.c:563:12: error: ‘nread’ may be used uninitialized [-Werror=maybe-uninitialized]
  563 |         if ((nread == -1) && (errno == EINTR)) {
      |            ^
../../lib/async_req/async_sock.c:531:17: note: ‘nread’ was declared here
  531 |         ssize_t nread, more;
      |                 ^~~~~
cc1: all warnings being treated as errors

Signed-off-by: Jennifer Sutton <jennifersutton@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>