ctdb-tests: Fix CTDB -O3 --picky-developer build on CentOS 7

ctdb-tests: Fix CTDB -O3 --picky-developer build on CentOS 7

gcc 4.8.5 complains:

[319/381] Compiling ctdb/tests/src/system_socket_test.c
../tests/src/system_socket_test.c: In function ‘test_tcp’:
../tests/src/system_socket_test.c:196:20: error: ‘rst_out’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  assert((rst != 0) == (rst_out != 0));
                    ^
cc1: all warnings being treated as errors

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Andreas Schneider <asn@samba.org>