gnu: python-libxml2, itstool: Provide crash-free variants.

Fixes <https://bugs.gnu.org/37468>.

* gnu/packages/patches/python-libxml2-utf8.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/xml.scm (python-libxml2/fixed): New variable.
* gnu/packages/glib.scm (itstool/fixed): New variable.
* gnu/packages/gnome.scm (gnumeric)[native-inputs]: Use ITSTOOL/FIXED
instead of ITSTOOL.

gnu/packages/patches/python-libxml2-utf8.patch

@@ -0,0 +1,40 @@
+This patch fixes a crash in the libxml2 bindings for Python 3.x
+that 'itstool' stumbles upon when processing UTF-8 data:
+
+  https://issues.guix.gnu.org/issue/37468
+
+Patch by Jan Matejek
+from <https://bugzilla.opensuse.org/show_bug.cgi?id=1065270>.
+
+--- libxml2-2.9.5.orig/python/libxml.c
++++ libxml2-2.9.5/python/libxml.c
+@@ -1620,6 +1620,7 @@ libxml_xmlErrorFuncHandler(ATTRIBUTE_UNU
+     PyObject *message;
+     PyObject *result;
+     char str[1000];
++    unsigned char *ptr = (unsigned char *)str;
+ 
+ #ifdef DEBUG_ERROR
+     printf("libxml_xmlErrorFuncHandler(%p, %s, ...) called\n", ctx, msg);
+@@ -1636,12 +1637,20 @@ libxml_xmlErrorFuncHandler(ATTRIBUTE_UNU
+ 	    str[999] = 0;
+         va_end(ap);
+ 
++#if PY_MAJOR_VERSION >= 3
++        /* Ensure the error string doesn't start at UTF8 continuation. */
++        while (*ptr && (*ptr & 0xc0) == 0x80)
++            ptr++;
++#endif
++
+         list = PyTuple_New(2);
+         PyTuple_SetItem(list, 0, libxml_xmlPythonErrorFuncCtxt);
+         Py_XINCREF(libxml_xmlPythonErrorFuncCtxt);
+-        message = libxml_charPtrConstWrap(str);
++        message = libxml_charPtrConstWrap(ptr);
+         PyTuple_SetItem(list, 1, message);
+         result = PyEval_CallObject(libxml_xmlPythonErrorFuncHandler, list);
++        /* Forget any errors caused in the error handler. */
++        PyErr_Clear();
+         Py_XDECREF(list);
+         Py_XDECREF(result);
+     }