(ITS#6403) crash with translucent + back-null

[h.b.furuseth@usit.uio.no]

Full_Name: Hallvard B Furuseth
Version: HEAD
OS: Linux
URL: 
Submission from: (NULL) (129.240.6.233)
Submitted by: hallvard


Callers of bi->bi_entry_get_rw(,,,,,Entry **ent) should set *ent = NULL.
Translucent doesn't. null_back_entry_get() assert()s it.  Crash.
Happens with ./run -b null test034-translucent.

I'll fix translucent for now, but I assume a saner fix would be to
change that "calling convention" - which I suspect exists in the first
place because I once gave a bug just like this the same minimal fix.