If the packpos returned by the read_index() call is 4, return immediately.
In this case, the very first transaction, which contains the root object,
is after the pack time.  This is a degenerate case, but it seems clearest
to do nothing.

This change eliminates an unexpected traceback in pack() that occurred
frequently, but not every time, when testing on Windows.  When
read_index() returned 4, a seek() call in _redundant_pack() would fail
with a bad argument.

Add an assertion to _redundant_pack() because it should never be called
with bogus arguments as a result of the other change.