Libunit: fixing read buffer leakage.

If shared queue is empty, allocated read buffer should be explicitly
released.

Found by Coverity (CID 363943).
The issue was introduced in f5ba5973a0a3.

src/nxt_unit.c

@@ -4948,6 +4948,7 @@ nxt_unit_dequeue_request(nxt_unit_ctx_t *ctx)
 
     rc = nxt_unit_app_queue_recv(lib->shared_port, rbuf);
     if (rc != NXT_UNIT_OK) {
+        nxt_unit_read_buf_release(ctx, rbuf);
         goto done;
     }