Fix error message for COMMENT/SECURITY LABEL ON COLUMN xxx IS 'yyy'

When the column name is an unqualified name, rather than table.column,
the error message complains about too many dotted names, which is
wrong.  Report by Peter Eisentraut based on examination of the
sepgsql regression test output, but the problem also affects COMMENT.
New wording as suggested by Tom Lane.

contrib/sepgsql/expected/label.out

@@ -65,7 +65,7 @@ SECURITY LABEL ON TABLE t2
 ERROR:  SELinux: invalid security label: "invalid security context"
 SECURITY LABEL ON COLUMN t2
     IS 'system_u:object_r:sepgsql_ro_table_t:s0';	-- be failed
-ERROR:  improper relation name (too many dotted names): 
+ERROR:  column name must be qualified
 SECURITY LABEL ON COLUMN t2.b
     IS 'system_u:object_r:sepgsql_ro_table_t:s0';	-- ok
 --

src/backend/catalog/objectaddress.c

@@ -526,6 +526,10 @@ get_object_address_attribute(ObjectType objtype, List *objname,
 	const char *attname;
 
 	/* Extract relation name and open relation. */
+	if (list_length(objname) < 2)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("column name must be qualified")));
 	attname = strVal(lfirst(list_tail(objname)));
 	relname = list_truncate(list_copy(objname), list_length(objname) - 1);
 	relation = relation_openrv(makeRangeVarFromNameList(relname), lockmode);

src/test/regress/input/security_label.source

@@ -44,6 +44,7 @@ SET SESSION AUTHORIZATION seclabel_user1;
 
 SECURITY LABEL ON TABLE seclabel_tbl1 IS 'classified';			-- OK
 SECURITY LABEL ON COLUMN seclabel_tbl1.a IS 'unclassified';		-- OK
+SECURITY LABEL ON COLUMN seclabel_tbl1 IS 'unclassified';	-- fail
 SECURITY LABEL ON TABLE seclabel_tbl1 IS '...invalid label...';	-- fail
 SECURITY LABEL FOR 'dummy' ON TABLE seclabel_tbl1 IS 'unclassified';	-- OK
 SECURITY LABEL FOR 'unknown_seclabel' ON TABLE seclabel_tbl1 IS 'classified';	-- fail

src/test/regress/output/security_label.source

@@ -37,6 +37,8 @@ LOAD '@abs_builddir@/dummy_seclabel@DLSUFFIX@';
 SET SESSION AUTHORIZATION seclabel_user1;
 SECURITY LABEL ON TABLE seclabel_tbl1 IS 'classified';			-- OK
 SECURITY LABEL ON COLUMN seclabel_tbl1.a IS 'unclassified';		-- OK
+SECURITY LABEL ON COLUMN seclabel_tbl1 IS 'unclassified';	-- fail
+ERROR:  column name must be qualified
 SECURITY LABEL ON TABLE seclabel_tbl1 IS '...invalid label...';	-- fail
 ERROR:  '...invalid label...' is not a valid security label
 SECURITY LABEL FOR 'dummy' ON TABLE seclabel_tbl1 IS 'unclassified';	-- OK