Fix min_heap_erase when we remove an element from the middle of the heap.

Fix min_heap_erase when we remove an element from the middle of the heap.

Previously, we could lose the heap property when we removed an item
whose parent was greater than the last element in the heap.  We would
replace the removed item with the last element, and consider shifting
it down, but we wouldn't consider shifting it up.

Patch from Marko Kreen.

svn:r1226