Bug in radix heap

[Peter Madarasi]

Levente Birszki reports that the implementation of radix heap does not work correctly in certain cases. It seems to mess up the sizes of the buckets, and it may pop an item with non-minimal key. For example,

const int n = 10; // max num items
const int c = 10; // max key

lemon::RangeMap<int> map(n, -1);
lemon::RadixHeap<lemon::RangeMap<int>> heap(map, 0, c);

heap.push(0, 2); // (index, priority)
heap.push(1, 3);
heap.push(2, 4);

heap.pop(); // ok, pops (0, 2)
heap.increase(1, 5);
heap.pop(); // oops, pops (1, 5) instead of (2,4)

Attached is a possible fix of the issue. The patch also extends heap_test.cc with the problematic example given above. The solution is based on a proposal of Levente.