BUG/MINOR: chunk: Allow large chunks uses from large const buffers

BUG/MINOR: chunk: Allow large chunks uses from large const buffers

When a chunk was retrieved via get_best_trash_chunk() or
alloc_best_trash_chunk(), it was not possible to get a large chunk when the
original buffer was a constant 0-sized buffer, independently on size of data
pointed by it.

With this patch, we relaxe the test. It is now possible to get a large
chunks from a 0-sized buffer if it points on large data. Indeed, in that
case, it means that data are hosted in a large buffer, so it is safe to get
a large chunk if possible.

This patch should be backported to 3.4 after a period of observation.