mtp: fix mishandling of huge track counts

the request made to the mtp library for the available tracks returns a linked list. a callback mechanism is additionally used for capturing the total track count.

problematically:

• this callback receives a uint64_t track count which was then stored in an int before being used by a calloc() call to allocate enough memory to store all of the corresponding items. the truncation may thus mean that we do not allocate enough memory if faced with a huge count.
• there was nothing to prevent the iteration of the linked list from going past the count, allowing the int based counter to wrap giving undefined behaviour.