http2: fix memory leak for uncommon headers

Fix a memory leak that occurs with header names that are
short and not present in the static table of default headers.

Backport-PR-URL: #22850
PR-URL: #21336
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Minwoo Jung <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>

src/node_http2.h

@@ -1299,6 +1299,7 @@ class ExternalHeader :
     }
 
     if (may_internalize && vec.len < 64) {
+      nghttp2_rcbuf_decref(buf);
       // This is a short header name, so there is a good chance V8 already has
       // it internalized.
       return GetInternalizedString(env, vec);