Always NUL terminate buf in OBJ_obj2txt()

Always NUL terminate buf in OBJ_obj2txt()

OBJ_obj2txt() is often called without error checking and is used for
reporting unexpected or malformed objects. As such, we should ensure
buf is a string even on failure. This had long been the case before it
was lost in a recent rewrite. If obj and obj->data are both non-NULL
this is already taken care of by i2t_ASN1_OBJECT_internal(), so many
callers were still safe.

ok miod