Suppress compiler warning in non-USE_LIBXML builds.

Compilers that don't realize that ereport(ERROR) doesn't return
complained that XmlTableGetValue() failed to return a value.

Also, make XmlTableFetchRow's non-USE_LIBXML case look more like
the other ones.  As coded, it could lead to "unreachable code"
warnings with USE_LIBXML enabled.

Oversights in commit fcec6caaf.  Per buildfarm.

src/backend/utils/adt/xml.c

@@ -4389,9 +4389,8 @@ XmlTableFetchRow(TableFuncScanState *state)
 	return false;
 #else
 	NO_XML_SUPPORT();
-#endif   /* not USE_LIBXML */
-
 	return false;
+#endif   /* not USE_LIBXML */
 }
 
 /*
@@ -4561,6 +4560,7 @@ XmlTableGetValue(TableFuncScanState *state, int colnum,
 	return result;
 #else
 	NO_XML_SUPPORT();
+	return 0;
 #endif   /* not USE_LIBXML */
 }