tests/unit/socket-helpers: Don't close(-1)

tests/unit/socket-helpers: Don't close(-1)

In socket_check_afunix_support() we call socket(PF_UNIX, SOCK_STREAM, 0)
to see if it works, but we call close() on the result whether it
worked or not. Only close the fd if the socket() call succeeded.
Spotted by Coverity.

Resolves: Coverity CID 1497481

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20240312183810.557768-3-peter.maydell@linaro.org