QUIC: fixed udp buffer initialization.

The start field is used to check if the QUIC packet is first in the datagram.
This fixes stateless reset detection.

src/event/ngx_event_udp.c

@@ -279,6 +279,8 @@ ngx_event_recvmsg(ngx_event_t *ev)
 
             buf.pos = buffer;
             buf.last = buffer + n;
+            buf.start = buf.pos;
+            buf.end = buffer + sizeof(buffer);
 
             rev = c->read;