BUG/MINOR: compression: consider the expansion factor in init

BUG/MINOR: compression: consider the expansion factor in init

When checking if the buffer is large enough, we used to rely on a fixed
size that was "apparently" enough. We need to consider the expansion
factor of deflate-encoded streams instead, which is of 5 bytes per 32kB.
The previous value was OK till 128kB buffers but became wrong past that.
It's totally harmless since we always keep the reserve when compressiong,
so there's 1kB or so available, which is enough for buffers as large as
6.5 MB, but better fix the check anyway.

This fix could be backported into 1.5 since compression was added there.