Fix UnionStmt toSql bug (#249)

Author: kangkaisen

fe/src/main/java/org/apache/doris/analysis/UnionStmt.java

@@ -534,7 +534,7 @@ public String toSql() {
         StringBuilder strBuilder = new StringBuilder();
         Preconditions.checkState(operands.size() > 0);
         strBuilder.append(operands.get(0).getQueryStmt().toSql());
-        for (int i = 1; i < operands.size() - i; ++i) {
+        for (int i = 1; i < operands.size() - 1; ++i) {
             strBuilder.append(
               " UNION " + ((operands.get(i).getQualifier() == Qualifier.ALL) ? "ALL " : ""));
             if (operands.get(i).getQueryStmt() instanceof UnionStmt) {