http2: fix premature destroy

Check stream._writableState.finished instead of stream.writable
as the latter can lead to premature calls to destroy and dropped
writes on busy processes.

PR-URL: #21051
Fixes: #20750
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/http2/core.js

@@ -1896,7 +1896,7 @@ class Http2Stream extends Duplex {
     }
 
     // TODO(mcollina): remove usage of _*State properties
-    if (!this.writable) {
+    if (this._writableState.finished) {
       if (!this.readable && this.closed) {
         this.destroy();
         return;