[6366] Added intval in order to prevent SQL injection to $id_shop_list in ObjectModel

Official description: #6366: Prevented SQL injections object model

This vulnerability affects the following application versions:

• PrestaShop 1.5.6.1
• PrestaShop 1.5.6.2
• PrestaShop 1.5.6.3
• PrestaShop 1.6.0.2
• PrestaShop 1.6.0.2 alpha 2
• PrestaShop 1.6.0.3
• PrestaShop 1.6.0.3 beta 1
• PrestaShop 1.6.0.4
• PrestaShop 1.6.0.4 RC1
• PrestaShop 1.6.0.5
• PrestaShop 1.6.0.6
• PrestaShop 1.6.0.7
• PrestaShop 1.6.0.8
• PrestaShop 1.6.0.9
• PrestaShop 1.6.0.10
• PrestaShop 1.6.0.11
• PrestaShop 1.6.0.12
• PrestaShop 1.6.0.13
• PrestaShop 1.6.0.14
• PrestaShop 1.6.1.0
• PrestaShop 1.6.1.0 RC4
• PrestaShop 1.6.1.0 RC5
• PrestaShop 1.6.1.1
• PrestaShop 1.6.1.1 RC1
• PrestaShop 1.6.1.1 RC2
• PrestaShop 1.6.1.2
• PrestaShop 1.6.1.2 RC1
• PrestaShop 1.6.1.2 RC2
• PrestaShop 1.6.1.2 RC3
• PrestaShop 1.6.1.2 RC4
• PrestaShop 1.6.1.3
• PrestaShop 1.6.1.3 RC1
• PrestaShop 1.6.1.4
• PrestaShop 1.6.1.5
• PrestaShop 1.6.1.6
• PrestaShop 1.6.1.7
• PrestaShop 1.6.1.8
• PrestaShop 1.6.1.9
• PrestaShop 1.6.1.10
• PrestaShop 1.6.1.11
• PrestaShop 1.6.1.11 beta 1
• PrestaShop 1.6.1.11-beta.1.0
• PrestaShop 1.6.1.12
• PrestaShop 1.6.1.13
• PrestaShop 1.6.1.14
• PrestaShop 1.6.1.15