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.

PR-URL: #21336
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

Author: addaleax

src/node_http2.h

@@ -1203,6 +1203,7 @@ 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);