net: use asserts in JS Socket Stream to catch races in future

Author: pimterry

lib/internal/js_stream_socket.js

@@ -149,6 +149,7 @@ class JSStreamSocket extends Socket {
     }
 
     const handle = this._handle;
+    assert(handle !== null);
 
     process.nextTick(() => {
       // Ensure that write is dispatched asynchronously.
@@ -181,6 +182,8 @@ class JSStreamSocket extends Socket {
     }
 
     const handle = this._handle;
+    assert(handle !== null);
+
     const self = this;
 
     let pending = bufs.length;