bus: when getting a kdbus connection into a container wait first for child, then read message

There's no EOF generated for AF_UNIX/SOCK_DGRAM sockets, hence let's
wait for the child first to see if it succeeded, only then read the socket.

src/machine/machinectl.c

@@ -451,6 +451,15 @@ static int openpt_in_namespace(pid_t pid, int flags) {
         close_nointr_nofail(pair[1]);
         pair[1] = -1;
 
+        r = wait_for_terminate(child, &si);
+        if (r < 0 || si.si_code != CLD_EXITED || si.si_status != EXIT_SUCCESS || master < 0) {
+
+                if (master >= 0)
+                        close_nointr_nofail(master);
+
+                return r < 0 ? r : -EIO;
+        }
+
         if (recvmsg(pair[0], &mh, MSG_NOSIGNAL|MSG_CMSG_CLOEXEC) < 0)
                 return -errno;
 
@@ -470,15 +479,6 @@ static int openpt_in_namespace(pid_t pid, int flags) {
                         master = fds[0];
                 }
 
-        r = wait_for_terminate(child, &si);
-        if (r < 0 || si.si_code != CLD_EXITED || si.si_status != EXIT_SUCCESS || master < 0) {
-
-                if (master >= 0)
-                        close_nointr_nofail(master);
-
-                return r < 0 ? r : -EIO;
-        }
-
         return master;
 }