The patch I attached actually frees memory before the connection ends but do a
lot of assumptions that might not work. The cleanest solution is probably just
to disable chunked transfer encoding with the option:

chunked_transfer_encoding off;

which removes the problem at the root (if it is ok for you to disable chunked
transfer encoding, of if you already implement it in the proxied server).

AFAIK nginx does not support memory freeing/reuse before connection ends and,
with current architecture, is not a trivial job to do (requires to handle reuse
of non-contiguous allocations).