Upstream: made the assignment more obvious.

No functional changes.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 4183d9d..ae71a11 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1701,7 +1701,7 @@
     n = u->buffer.last - u->buffer.pos;
 
     if (n) {
-        u->buffer.last -= n;
+        u->buffer.last = u->buffer.pos;
 
         u->state->response_length += n;