Fix BIO_set_nbio_accept()

The code that implements this control would work when enabling nbio,
but the disabling code needed fixing.
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/bio/bss_acpt.c

@@ -474,16 +474,17 @@ static long acpt_ctrl(BIO *b, int cmd, long num, void *ptr)
                 data->param_serv = BUF_strdup(ptr);
                 b->init = 1;
             } else if (num == 2) {
-                if (ptr != NULL)
-                    data->bind_mode |= BIO_SOCK_NONBLOCK;
-                else
-                    data->bind_mode &= ~BIO_SOCK_NONBLOCK;
+                data->bind_mode |= BIO_SOCK_NONBLOCK;
             } else if (num == 3) {
                 BIO_free(data->bio_chain);
                 data->bio_chain = (BIO *)ptr;
             } else if (num == 4) {
                 data->accept_family = *(int *)ptr;
             }
+        } else {
+            if (num == 2) {
+                data->bind_mode &= ~BIO_SOCK_NONBLOCK;
+            }
         }
         break;
     case BIO_C_SET_NBIO: