Previously, bzip2_decode_buffer and deflate_decode_buffer left
*new_len unchanged if the compressed input data ended unexpectedly.
This behaviour was also inherited by decode_encoded_buffer,
whose only caller render_encoded_document preinitializes the variable
and so did not crash.
With this change, the functions now store in *new_len the number of
bytes that were successfully decoded, even if more bytes were expected.
An error should perhaps be reported to the user, but I don't think the
previous version did that either, as it returned a non-NULL pointer.
This function was unused and only the dummy backend implemented it
correctly. Fixing bug 534 will probably require adding this function
back again, but I think it will be easier to do it from scratch than
fix the bogus implementations we had here.
