Added Validation rule to Specific price

Line 59: Changing the id_currency to validate as isUnsignedId prevents the id_currency being set as a negative number for a non existent Currency ID.

This change brings the validation for this file in line with the rest of PrestaShop core when it comes to handling ID numbers
Same changes as Specific Price
Merge pull request #2266 from davidsivocha/patch-1

[-] CORE : Added Validation rule to Specific price id_currency