Fix bug at fork on FreeBSD

The main thread in a forked process appears not to own the read-write
lock.

process.c

@@ -4234,7 +4234,13 @@ rb_fork_ruby(int *status)
         child.error = err = errno;
 
         disable_child_handler_fork_parent(&old); /* yes, bad name */
-        rb_thread_release_fork_lock();
+        if (
+#if defined(__FreeBSD__)
+            pid != 0 &&
+#endif
+            true) {
+            rb_thread_release_fork_lock();
+        }
         if (pid == 0) {
             rb_thread_reset_fork_lock();
         }