Update nxt_conn_write.c

Fix Coverity report:
CID 200492 (nginx#1 of 1): Uninitialized scalar variable (UNINIT)2. uninit_use_in_call: Using uninitialized value sb.last when calling nxt_sendbuf_mem_coalesce. [show details]

Author: RanierV

src/nxt_conn_write.c

@@ -322,6 +322,7 @@ nxt_event_conn_io_sendbuf(nxt_conn_t *c, nxt_buf_t *b, size_t limit)
     sb.iobuf = iob;
     sb.nmax = NXT_IOBUF_MAX;
     sb.sync = 0;
+    sb.last = 0;
     sb.size = 0;
     sb.limit = limit;