Bugfix: might cause segfault in dyanmic resolve

*) set rrp->dyn_peers to null in round robin create dynamic peers
*) check if rrp is null in finalize upstream request to avoid segfault
   if reqeust is finalized before rrp is created for r->upstream->peer
*) set rrp->dyn_peers to null before re-assign dyn_peers to new dynamic
   peers

Signed-off-by: paulyang <paulyang.inf@gmail.com>