subqueue_load() could yield "Notice: Undefined variable: subqueues"

subqueue_load() can yield an "Notice: Undefined variable: subqueues" if it is called with an empty array of sqid's.

Although are rare situation we experienced this when interfacing to the nodequeue module.

The fix is to initialize $subqueues to an empty array in the function.

I have attached a patch that fixes this an improves the function documentation a bit (added return value and adhere to current coding standards).