Fix datarace reported by valgrind/helgrind

This doesn't really fix the datarace but changes it so it can only happens
once. This isn't really a problem since we always just set it to the same
value. We now just stop writing it after the first time.

PR3584, https://bugs.debian.org/534534



Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Rich Salz <rsalz@openssl.org>