dir/server/tree: check log entry size for large values
dir/server/tree: check log entry size for large values

Calling make([]byte, n) for large values of n will trigger a panic,
and invalid log entries may provide such large values of n.

This was found while tracking down some flakiness in our tests:
	https://travis-ci.org/upspin/upspin/builds/226568653
The flakiness remains a mystery, but this check is better than a panic.

Change-Id: I3f91058959d9290603479b29bf129ec150e9e6ff
Reviewed-on: https://upspin-review.googlesource.com/9360
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin