Disabling HAMT sharing for directories leads to nil pointer dereference

[RubenKelevra]

Version information:

go-ipfs version: 0.4.23-6ce9a355f
Repo version: 7
System version: amd64/linux
Golang version: go1.13.7

Description:

I've disabled HAMT sharding for directories on a node that had it previously activated. This results in trouble reading the repo.

Expected behavior

Detect that there's HAMT data and read them anyway, but do not create new HAMT sharded directories

Log

crash_hamt.log

[Stebalien]

Those crashes are actually from trying to write to a HAMT directory. When sharding is disabled, we won't create new sharded directories but we'll still modify existing ones when requested through MFS.

This is #6860.