protobuf-c: parse_required_member(): zero bd->data in 0-length byte string case

in the case that we are unpacking a 0-length byte string, we need to
explicitly set bd->data to NULL, otherwise we may try to free() a stray
pointer when protobuf_c_message_free_unpacked() is called on the
containing message.

(Issue #157.)
bump version to 1.0.1
ChangeLog: 1.0.1