When calling xmlIOParseDTD(), a new xmlParserCtxt is
build and xmlNewIOInputStream() is called with that
context, the given xmlParserInputBufferPtr and encoding.
If the encoding is unequal to XML_CHAR_ENCODING_NONE,
xmlSwitchEncoding() and than xmlSwitchToEncoding() is called,
again with the new parser context and the appropriate
encoding handler (if found).
xmlSwitchToEncoding() now checks (ctxt->input != NULL)
and calls
xmlErrInternal(ctxt, "xmlSwitchToEncoding : no input\n",
	                   NULL);
since the newly build context obviously has no input
pointer.
xmlErrInternal() sets ctxt->wellFormed = 0 and after
xmlIOParseDTD() has called xmlParseExternalSubset(),
it checks (ctxt->wellFormed) and returns NULL.

Some debugging in v2.5.4 and v2.6.1 shows that switching
to the given encoding fails in both versions due to the
check of (ctxt->input != NULL), but only v2.6.1 sets
ctxt->wellFormed = 0.

As a result it's not possible to parse in a DTD
using xmlIOParseDTD() with a character encoding
unequal to XML_CHAR_ENCODING_NONE.