socket: fix use of ERRNO_IS_DISCONNECT()

Given that ERRNO_IS_DISCONNECT() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_DISCONNECT() is the value
returned by socket_acquire_peer() which can legitimately return 1
without errno semantics, so fix this by moving ERRNO_IS_DISCONNECT()
invocation to the branch where the return value is known to be negative.

(cherry picked from commit d5f8890)

Author: ldv-alt

src/core/socket.c

@@ -2361,10 +2361,12 @@ static void socket_enter_running(Socket *s, int cfd_in) {
 
                 if (s->max_connections_per_source > 0) {
                         r = socket_acquire_peer(s, cfd, &p);
-                        if (ERRNO_IS_DISCONNECT(r))
-                                return;
-                        if (r < 0) /* We didn't have enough resources to acquire peer information, let's fail. */
+                        if (r < 0) {
+                                if (ERRNO_IS_DISCONNECT(r))
+                                        return;
+                                /* We didn't have enough resources to acquire peer information, let's fail. */
                                 goto fail;
+                        }
                         if (r > 0 && p->n_ref > s->max_connections_per_source) {
                                 _cleanup_free_ char *t = NULL;