xml.c: do not convert UTF-16 surrogate pairs to an invalid sequence

Simple way to reproduce:

	printf '<item><title>&#xdc00;</title></item>' | sfeed | iconv -t utf-8

Result:
	iconv: (stdin):1:8: cannot convert

Output result:

	printf '<item><title>&#xdc00;</title></item>' | sfeed

Before:

00000000  09 ed b0 80 09 09 09 09  09 09 09 0a              |............|
0000000c

After:

00000000  09 26 23 78 64 63 30 30  3b 09 09 09 09 09 09 09  |.&#xdc00;.......|
00000010  0a                                                |.|
00000011

The entity is output as a literal string. This allows to see more easily whats
wrong and debug the feed and it is consistent with the current behaviour of
invalid named entities (&bla;). An alternative could be a UTF-8 replacement
symbol (codepoint 0xfffd).

Reference: https://unicode.org/faq/utf_bom.html , specificly:

"Q: How do I convert an unpaired UTF-16 surrogate to UTF-8? "
"A: A different issue arises if an unpaired surrogate is encountered when
converting ill-formed UTF-16 data. By representing such an unpaired surrogate
on its own as a 3-byte sequence, the resulting UTF-8 data stream would become
ill-formed. While it faithfully reflects the nature of the input, Unicode
conformance requires that encoding form conversion always results in a valid
data stream. Therefore a converter must treat this as an error. [AF]"

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