Do not write out terminating NUL in i2a_ASN1_OBJECT()

Do not write out terminating NUL in i2a_ASN1_OBJECT()

The conversion to CBB made us write out an extra NUL since we no longer
use the return value of i2t_ASN1_OBJECT() (which returns strlen(data))
but rather the size of the CBB (which includes a terminal NUL) to write
out data.

Issue found by anton via an openssl-ruby test failure.

ok jsing