Shopping cart with a value less than 1

Hello! I have a shopping cart with 2 images, one to enlarge and one to decrease the quantity. Everything works fine (it increases the quantity and updates the subtotal and tb the total. The same happens when I decrease.) I have a button to remove each item, which works perfectly. The problem is that the amount can not be less than 1, as it decreases (0, -1, -2 ....) and updates the values with negative value. Anyway, I need the quantity not less than 1 or, if it is, that the item be removed. Thank you in advance for being able to help me.

Follow the code:    

<?php

    require ("conexao.php");

    if(count($_SESSION['valida_carrinho']) == 0){

       echo "
             <table style='background-color:#EEEEEE; width:94%; height:40px; text-align:center; margin-top:10px; margin-left:40px; font-size:15px;'>
                 <tr>
                   <td><strong>Não há produtos no carrinho.</strong></td>                
                 </tr>
             </table>";

       }else{           

       $total = 0;

       foreach($_SESSION['valida_carrinho'] as $id => $quantidade){

         $sql = "SELECT * FROM produtos WHERE id = '$id'";
         $query = mysqli_query ($conn, $sql)or die (mysqli_error());
         $line = mysqli_fetch_assoc($query);

         $imagem = $line['imagem'];
         $nome = $line['nome'];
         $preco = $line['preco'];
         $subtotal = $line['preco'] * $quantidade;
         $total = $total += $subtotal;  
         $_SESSION['total'] = $total;            

         echo'
           <table>

            <tr><td><img src ="imagens/'.$line['imagem'].'"/></td>
            <tr><td>'.$nome.'</td></tr>
            <tr><td>R$ '.number_format($preco, 2, ',', '.').'</td></tr>
            <tr>
            <td><a href="?acao=sub&id='.$id.'"><img src = "imagens/negativo.png"></a>
            <input id= "product" type = "text" name = "product['.$id.']" value = "'.$quantidade.'"style="text-align:center; width:25px; height:25px;"/></a>
            <a href="?acao=add&id='.$id.'"><img src = "imagens/positivo.png"></a></td>
            </tr>                         
            <tr><td><a href="?acao=del&id='.$id.'">Remover</a></td>

           </table>';       

       }