Upstream keepalive: clearing of c->data in cached connections.

Previously, connections returned from keepalive cache had c->data
pointing to the keepalive cache item.  While this shouldn't be a problem
for correct code, as c->data is not expected to be used before it is set,
explicitly clearing it might help to avoid confusion.

src/http/modules/ngx_http_upstream_keepalive_module.c

@@ -275,6 +275,7 @@ found:
 
     c->idle = 0;
     c->sent = 0;
+    c->data = NULL;
     c->log = pc->log;
     c->read->log = pc->log;
     c->write->log = pc->log;