Stream: fixed potential error log buffer overrun.

Found by Duan Jiong <djduanjiong@gmail.com>.

src/stream/ngx_stream_handler.c

diff --git a/src/stream/ngx_stream_handler.c b/src/stream/ngx_stream_handler.c
index 24afbcd..b3edb68 100644
--- a/src/stream/ngx_stream_handler.c
+++ b/src/stream/ngx_stream_handler.c
@@ -328,9 +328,11 @@
     p = ngx_snprintf(buf, len, ", client: %V, server: %V",
                      &s->connection->addr_text,
                      &s->connection->listening->addr_text);
+    len -= p - buf;
+    buf = p;
 
     if (s->log_handler) {
-        return s->log_handler(log, p, len);
+        p = s->log_handler(log, buf, len);
     }
 
     return p;