Handle unterminated strings when encoding.

If the null terminator is not present, string will be limited to the
data size of the field.

If you are still using the pb_enc_string (deprecated since 0.1.3) from
callbacks, now would be an excellent time to stop. The pb_field_t for
the callback will not contain proper data_size. Use pb_encode_string()
instead.

Update issue 68
Status: FixedInGit