[CLEANUP] buffers: remove remains of wrong obsolete length check

[CLEANUP] buffers: remove remains of wrong obsolete length check

A check was performed in buffer_replace2() to compare buffer
length with its read pointer. This has been wrong for a long
time, though it only has an impact when dealing with keep-alive
requests/responses. In theory this should be backported but
the check has no impact without keep-alive.
(cherry picked from commit 43a7e6620b79e0e771dbaf2a60b57c96d9ba60e5)