Fix a horrible BN bug in bn_expand2 which caused BN_add_word() et al to fail

Fix a horrible BN bug in bn_expand2 which caused BN_add_word() et al to fail
when they cause the destination to expand.

To see how evil this is try this:

#include <pem.h>
main()
{
	BIGNUM *bn = NULL;
        int i;
	bn = BN_new();
	BN_hex2bn(&bn, "FFFFFFFF");
	BN_add_word(bn, 1);
	printf("Value %s\n", BN_bn2hex(bn));
}

This would typically fail before the patch.

It also screws up if you comment out the BN_hex2bn line above or in any
situation where BN_add_word() causes the number of BN_ULONGs in the result
to change (try doubling the number of FFs).