Fixing shared memory thread safety issue.

Do not reuse shared memory segment with different port until this segment
successfully received and indexed on other side. However, segment can be used
to transfer data via the port it was sent at any time.

src/nxt_port_memory_int.h

@@ -49,6 +49,7 @@ typedef nxt_atomic_uint_t  nxt_free_map_t;
 struct nxt_port_mmap_header_s {
     uint32_t        id;
     nxt_pid_t       pid; /* For sanity check. */
+    nxt_port_id_t   sent_over;
     nxt_free_map_t  free_map[MAX_FREE_IDX];
 };