Fix the ephemeron used to simulate and temporary device for the memory back-end of Git

[dinosaure]

Spotted by @ulugbekna, I need to implement some tests.

[ulugbekna]

I think this causes

index out of bounds
          Raised by primitive operation at file "src/carton/idx.ml" (inlined), line 43, characters 29-48
          Called from file "src/carton/idx.ml", line 64, characters 10-27
          Called from file "src/git/mem.ml", line 322, characters 6-223
          Called from file "src/git/sync.ml", line 232, characters 8-64
          Called from file "src/core/lwt.ml", line 1867, characters 23-26

with payload empty_repository_fetch in smart/test.ml

[ulugbekna]

To note: the same payload doesn't cause problems in git-unix

[dinosaure]

After investigation, we got an error about IDX temporary file because we did the projection of it before to do anything on it - I mean, we want to get contents of the file before to do receive and analyze a PACK file. So, the case should not appear - and it's not a bug as is (when such usage should not happen).

However, the patch assert that in any context, we have, at least, an empty contents. It does not change anything for us but, with this patch. the code looks similar to Cstruct_append (which is better). Let's start to merge.