Reuse tmpid buffer when getting the offset.

Reuse tmpid buffer when getting the offset.

There is no reason to allocate a 4 byte buffer for the offset
byteorder conversion when we have a perfectly useless 20 byte
buffer still available that was used to perform object ID
comparsion during the index search.  This reduces the amount
of memory required per object lookup by about 12 bytes on a
32 bit JVM, and that's a good reduction to make.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>