Avoid failure when selecting a namespace node in XMLTABLE.

It appears that libxml2 doesn't bother to set the "children" field of
an XML_NAMESPACE_DECL node to null; that field just contains garbage.
In v10 and v11, this can result in a crash in XMLTABLE().  The rewrite
done in commit 251cf2e27 fixed this, somewhat accidentally, in v12.
We're not going to back-patch 251cf2e27, however.  The case apparently
doesn't have wide use, so rather than risk introducing other problems,
just add a safety check to throw an error.

Even though no bug manifests in v12/HEAD, add the relevant test case
there too, to prevent future regressions.

Chapman Flack (per private report)

src/backend/utils/adt/xml.c

@@ -4612,6 +4612,12 @@ XmlTableGetValue(TableFuncScanState *state, int colnum,
 				xmlChar    *str;
 				xmlNodePtr	node;
 
+				node = xpathobj->nodesetval->nodeTab[0];
+				if (node->type == XML_NAMESPACE_DECL)
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("XMLTABLE cannot cast a namespace node to a non-XML result type")));
+
 				/*
 				 * Most nodes (elements and even attributes) store their data
 				 * in children nodes. If they don't have children nodes, it
@@ -4619,7 +4625,6 @@ XmlTableGetValue(TableFuncScanState *state, int colnum,
 				 * CDATA sections are an exception: they don't have children
 				 * but have content in the Text/CDATA node itself.
 				 */
-				node = xpathobj->nodesetval->nodeTab[0];
 				if (node->type != XML_CDATA_SECTION_NODE &&
 					node->type != XML_TEXT_NODE)
 					node = node->xmlChildrenNode;