Fix a regression in 2.9.12 where serializing empty HTML documents would
not add a terminating newline.

  https://gitlab.gnome.org/GNOME/libxml2/-/issues/266

Taken from upstream:

  https://gitlab.gnome.org/GNOME/libxml2/-/commit/92d9ab4c28842a09ca2b76d3ff2f933e01b6cd6f

diff --git a/HTMLtree.c b/HTMLtree.c
--- a/HTMLtree.c
+++ b/HTMLtree.c
@@ -763,11 +763,15 @@ htmlNodeDumpFormatOutput(xmlOutputBufferPtr buf, xmlDocPtr doc,
             if (((xmlDocPtr) cur)->intSubset != NULL) {
                 htmlDtdDumpOutput(buf, (xmlDocPtr) cur, NULL);
             }
-            /* Always validate cur->parent when descending. */
-            if ((cur->parent == parent) && (cur->children != NULL)) {
-                parent = cur;
-                cur = cur->children;
-                continue;
+            if (cur->children != NULL) {
+                /* Always validate cur->parent when descending. */
+                if (cur->parent == parent) {
+                    parent = cur;
+                    cur = cur->children;
+                    continue;
+                }
+            } else {
+                xmlOutputBufferWriteString(buf, "\n");
             }
             break;
 
><a href='/guix/tree/tests/base16.scm?id=276626ce8bebd8ee7643691d883a9aea069311d0'>tree</a><a href='/guix/commit/tests/base16.scm?id=276626ce8bebd8ee7643691d883a9aea069311d0'>commit</a><a href='/guix/diff/tests/base16.scm?id=276626ce8bebd8ee7643691d883a9aea069311d0'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/base16.scm'>
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<div class='path'>path: <a href='/guix/log/?id=276626ce8bebd8ee7643691d883a9aea069311d0'>root</a>/<a href='/guix/log/tests?id=276626ce8bebd8ee7643691d883a9aea069311d0'>tests</a>/<a href='/guix/log/tests/base16.scm?id=276626ce8bebd8ee7643691d883a9aea069311d0'>base16.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/base16.scm?id=276626ce8bebd8ee7643691d883a9aea069311d0&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-03-16 22:50:14 +0100'>2017-03-16</span></td><td><a href='/guix/commit/tests/base16.scm?id=4c0c4db0702048488a9712dbba7cad862c667d54'>utils: Move base16 procedures to (guix base16).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/utils.scm (bytevector-&gt;base16-string, base16-string-&gt;bytevector):
Move to...
* guix/base16.scm: ... here.  New file.
* tests/utils.scm ("bytevector-&gt;base16-string-&gt;bytevector"): Move to...
* tests/base16.scm: ... here.  New file.
* Makefile.am (MODULES): Add guix/base16.scm.
(SCM_TESTS): Add tests/base16.scm.
* build-aux/download.scm, guix/derivations.scm,
guix/docker.scm, guix/import/snix.scm, guix/pk-crypto.scm,
guix/scripts/authenticate.scm, guix/scripts/download.scm,
guix/scripts/hash.scm, guix/store.scm, tests/hash.scm,
tests/pk-crypto.scm: Adjust imports accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>