fix external_db_id definition in xref table

sql/patch_65_66_e.sql

+# patch_65_66_e.sql
+#
+# Title: Make xref.external_db_id INTEGER UNSIGNED NOT NULL
+#
+# Description:
+# Add NOT NULL to definition of external_db_id
+ALTER TABLE xref                    MODIFY external_db_id INTEGER UNSIGNED NOT NULL;
+
+# Patch identifier:
+INSERT INTO meta (species_id, meta_key, meta_value)
+  VALUES (NULL, 'patch', 'patch_65_66_e.sql|fix_external_db_id_in_xref');