Fix: nil pointer check for instance

[pratikmota]

Problem:

• foundImage is Pointer of civogo.Instance
• In case if GetOk() is not satisfied then both conditions will fail. foundImage pointer will remain nil
• If the pointer is nil then it will give a memory access violation for using foundImage.XXX properties.

Solution:

• If we are using pointer then need to check for nil value
• If nil value found then return error

My Changes:

• Checking foundImage pointer value is nil or not
• if nil is found then return an error for dataSourceInstanceRead function
• Spelling mistake correction: retrive --> retrieve