回退上一提交，处理index=-1的情形

以下方法中，如果x.getIndex() == -1 应如何处理请原作者解答一下

`
public boolean visit(SQLVariantRefExpr x)
`

src/main/java/com/alibaba/druid/sql/dialect/oracle/visitor/OracleExportParameterVisitor.java

@@ -46,7 +46,7 @@ public class OracleExportParameterVisitor extends OracleParameterizedOutputVisit
     }
 
     public OracleExportParameterVisitor(List<Object> parameters){
-        this(parameters,new StringBuilder(),true);
+        this(parameters,new StringBuilder(),false);
     }
 
     public OracleExportParameterVisitor(final Appendable appender) {

src/main/java/com/alibaba/druid/sql/dialect/postgresql/visitor/PGExportParameterVisitor.java

@@ -46,7 +46,7 @@ public class PGExportParameterVisitor extends PGParameterizedOutputVisitor imple
     }
 
     public PGExportParameterVisitor(final List<Object> parameters){
-        this(parameters,new StringBuilder(),true);
+        this(parameters,new StringBuilder(),false);
     }
 
     public PGExportParameterVisitor(final Appendable appender) {

src/main/java/com/alibaba/druid/sql/dialect/sqlserver/visitor/MSSQLServerExportParameterVisitor.java

@@ -46,7 +46,7 @@ public class MSSQLServerExportParameterVisitor extends SQLServerParameterizedOut
     }
 
     public MSSQLServerExportParameterVisitor(final List<Object> parameters){
-        this(parameters,new StringBuilder(),true);
+        this(parameters,new StringBuilder(),false);
     }
 
     public MSSQLServerExportParameterVisitor(final Appendable appender) {

src/main/java/com/alibaba/druid/sql/visitor/SQLASTOutputVisitor.java

@@ -845,7 +845,7 @@ public class SQLASTOutputVisitor extends SQLASTVisitorAdapter implements Printab
     public boolean visit(SQLVariantRefExpr x) {
         int index = x.getIndex();
 
-        if (parameters == null || index >= parameters.size()) {
+        if (index < 0 || parameters == null || index >= parameters.size()) {
             print0(x.getName());
             return false;
         }