path->ioctx == NULL crash for rename after create

[andrewchambers]

starting diod [-f -l 127.0.0.1:34321 -e /run/user/1000/TestDotlRename3873657423/001 -d 1 -n -U ac]
diod: P9_TVERSION tag 65535 msize 4096 version '9P2000.L'
diod: P9_RVERSION tag 65535 msize 4096 version '9P2000.L'
diod: P9_TATTACH tag 0 fid 0 afid -1 uname 'ac' aname '/run/user/1000/TestDotlRename3873657423/001' n_uname P9_NONUNAME
diod: P9_RATTACH tag 0 qid (000000000017e13c 0 'd')
diod: P9_TWALK tag 0 fid 0 newfid 1 nwname 0
diod: P9_RWALK tag 0 nwqid 0 
diod: P9_TLCREATE tag 0 fid 1 name 'x' flags 0x0 mode 00 gid 0
diod: P9_RLCREATE tag 0 qid (000000000017e13d 0 '') iounit 0
diod: P9_TRENAME tag 0 fid 1 dfid 0 name 'y'
diod: assertion failure: ioctx.c:448: path->ioctx == NULL

In this test I create a new file with LCREATE, then immediately try to rename it with TRENAME.
The protocol documentation says after LCREATE the fid points to the new item, but this causes a failure.

I think most existing clients tend to walk a new fid and avoid combining these operations hence we don't normally see this problem.