Use key accessors when checking competing foreign key definitions

Signed-off-by: minleejae <mmj9808@gmail.com>

Author: minleejae

src/main/java/net/sf/jsqlparser/statement/alter/AlterExpression.java

@@ -390,7 +390,8 @@ private ForeignKeyConstraint getForeignKeyConstraint() {
     private ForeignKeyConstraint foreignKeyForUpdate(boolean create) {
         ForeignKeyConstraint foreignKey = getForeignKeyConstraint();
         if (foreignKey == null && create) {
-            if (constraint != null || index != null || pkColumns != null || ukColumns != null) {
+            if (constraint != null || index != null || getPkColumns() != null
+                    || getUkColumns() != null) {
                 throw new IllegalStateException(
                         "Foreign-key setters cannot replace an existing definition; use setConstraint");
             }