io.c (novl_wait_for_single_fd): VM_UNREACHABLE instead of assert

This respects VM_CHECK_MODE and is more consistent with
the rest of our code.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65777 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -10725,7 +10725,7 @@ nogvl_wait_for_single_fd(int fd, short events)
         ret = rb_fd_select(fd + 1, 0, &fds, 0, 0);
         break;
       default:
-        assert(0 && "not supported yet, should never get here");
+        VM_UNREACHABLE(nogvl_wait_for_single_fd);
     }
 
     rb_fd_term(&fds);