MdeModulePkg/HiiDatabaseDxe: Add ASSERT before using the pointer 'String'
The 'Sting' is returned by the function GetUnicodeStringTextAndSize.
If it is NULL, function GetUnicodeStringTextAndSize will return
EFI_OUT_OF_RESOURCES, and error handling codes will cover it.
So the pointer 'Sting' can not be NULL when using it.
So we can add the ASSERT codes.