Fix OOB read in TS_OBJ_print_bio().

TS_OBJ_print_bio() misuses OBJ_txt2obj: it should print the result
as a null terminated buffer. The length value returned is the total
length the complete text reprsentation would need not the amount of
data written.

CVE-2016-2180

Thanks to Shi Lei for reporting this bug.

Reviewed-by: Matt Caswell <matt@openssl.org>