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 251cf2e2 fixed this, somewhat accidentally, in v12.
We're not going to back-patch 251cf2e2, 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/test/regress/expected/xml.out

@@ -1170,6 +1170,14 @@ SELECT * FROM XMLTABLE(XMLNAMESPACES(DEFAULT 'http://x.y'),
                       PASSING '<rows xmlns="http://x.y"><row><a>10</a></row></rows>'
                       COLUMNS a int PATH 'a');
 ERROR:  DEFAULT namespace is not supported
+SELECT * FROM XMLTABLE('.'
+                       PASSING '<foo/>'
+                       COLUMNS a text PATH 'foo/namespace::node()');
+                  a                   
+--------------------------------------
+ http://www.w3.org/XML/1998/namespace
+(1 row)
+
 -- used in prepare statements
 PREPARE pp AS
 SELECT  xmltable.*

src/test/regress/expected/xml_1.out

@@ -1048,6 +1048,14 @@ LINE 3:                       PASSING '<rows xmlns="http://x.y"><row...
                                       ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT * FROM XMLTABLE('.'
+                       PASSING '<foo/>'
+                       COLUMNS a text PATH 'foo/namespace::node()');
+ERROR:  unsupported XML feature
+LINE 2:                        PASSING '<foo/>'
+                                       ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
 -- used in prepare statements
 PREPARE pp AS
 SELECT  xmltable.*

src/test/regress/expected/xml_2.out

@@ -1150,6 +1150,14 @@ SELECT * FROM XMLTABLE(XMLNAMESPACES(DEFAULT 'http://x.y'),
                       PASSING '<rows xmlns="http://x.y"><row><a>10</a></row></rows>'
                       COLUMNS a int PATH 'a');
 ERROR:  DEFAULT namespace is not supported
+SELECT * FROM XMLTABLE('.'
+                       PASSING '<foo/>'
+                       COLUMNS a text PATH 'foo/namespace::node()');
+                  a                   
+--------------------------------------
+ http://www.w3.org/XML/1998/namespace
+(1 row)
+
 -- used in prepare statements
 PREPARE pp AS
 SELECT  xmltable.*

src/test/regress/sql/xml.sql

@@ -402,6 +402,10 @@ SELECT * FROM XMLTABLE(XMLNAMESPACES(DEFAULT 'http://x.y'),
                       PASSING '<rows xmlns="http://x.y"><row><a>10</a></row></rows>'
                       COLUMNS a int PATH 'a');
 
+SELECT * FROM XMLTABLE('.'
+                       PASSING '<foo/>'
+                       COLUMNS a text PATH 'foo/namespace::node()');
+
 -- used in prepare statements
 PREPARE pp AS
 SELECT  xmltable.*