i2d_ASN1_BOOLEAN(): allocate memory if the user didn't provide a buffer

Just as was done recently for i2d_ASN1_OBJECT, we also make
i2d_ASN1_BOOLEAN comply with the documentation.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6943)