With the patch applied to ENCODING_IMP, the test case gives the correct
output.  So it looks like making this change will fix the problem with invalid
UTF-8 strings in estudio.

I didn't test any other conversions - just 646 to UTF-32.  Someone should
write an eweasel test that converts various strings between different
encodings and compares `last_converted_stream' to different values,
depending on whether current platform is big endian or little endian, to
see if they are correct.