BUG/MAJOR: htx: Return the good block address after a defrag

BUG/MAJOR: htx: Return the good block address after a defrag

When an HTX structure is defragmented, it is possible to retrieve the new block
corresponding to an old one. This is useful to do a defrag during a loop on
blocks, to be sure to continue looping on the good block. But, instead of
returning the address of the new block in the HTX structure, the one in the
temporary structure used to do the defrag was returned, leading to unexpected
behaviours.

This patch must be backported to 1.9.