get_by_index

Get a IndexEntry by index.

Note that the returned IndexEntry is _only_ valid as long as:

1) The associated index has been closed 2) The entry has not been removed (see remove) 3) The index has not been refreshed (see read)

Changes to the IndexEntry will be reflected in the index once written back to disk using write.

Parameters:

this	a IndexEntries.
idx	the index of the entry.

Returns:

a IndexEntry or null if out of bounds.