BUG/MINOR: htx: Fix free space addresses calculation during a block expansion

BUG/MINOR: htx: Fix free space addresses calculation during a block expansion

When the payload of a block is shrinked or enlarged, addresses of the free
spaces must be updated. There are many possible cases. One of them is
buggy. When there is only one block in the HTX message and its payload is just
before the tail room and it needs to be moved in the head room to be enlarged,
addresses are not correctly updated. This bug may be hit by the compression
filter.

This patch must be backported to 2.0.