reqs: fix CID 1130969 - unchecked return code from library

Effectively, the return code of fcntl was not checked
by not checking the return code of socket_nonblocking()
for the server fd.

Found by coverity.

Signed-off-by: Michael Adam <obnox@samba.org>
(cherry picked from commit 68bd0b61b5)

src/reqs.c

@@ -1126,7 +1126,12 @@ static void relay_connection (struct conn_s *connptr)
                 return;
         }
 
-        socket_nonblocking (connptr->server_fd);
+        ret = socket_nonblocking (connptr->server_fd);
+        if (ret != 0) {
+                log_message(LOG_ERR, "Failed to set the server socket "
+                            "to non-blocking: %s", strerror(errno));
+                return;
+        }
 
         last_access = time (NULL);