Panicked while calling peek on a empty PriorityQueue #3
Description
https://github.com/garro95/priority-queue/blob/master/src/pqueue.rs#L90
thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', /Users/travis/build/rust-lang/rust/src/libcollections/vec.rs:1552