safer TAPP_destroy_tensor_info

[janbrandejs]

TAPP_destroy_tensor_info (and similar) can be made safer if it sets the pointer value (TAPP_tensor_info* info) to zero after if deallocates TAPP_tensor_info. This provides a way for the user to check whether TAPP_tensor_info was already deallocated before calling TAPP_destroy_tensor_info.
(suggestion by Anthony Scemama)

[devinamatthews]

I fixed this for TAPP_tensor_info in #20, I will check this issue across the interface now.