deps: cherry-pick eb306f463e from nghttp2 upstream

Original commit message:

  lib: add nghttp2_rcbuf_is_static()

  Add a `nghttp2_rcbuf_is_static()` method to tell whether a rcbuf
  is statically allocated.

  This can be useful for language bindings that wish to avoid
  creating duplicate strings for these buffers; concretely, I am
  planning to use this in the Node HTTP/2 module that is being
  introduced.

Ref: nghttp2/nghttp2@eb306f4
PR-URL: #14808
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

deps/nghttp2/lib/includes/nghttp2/nghttp2.h

@@ -469,6 +469,15 @@ NGHTTP2_EXTERN void nghttp2_rcbuf_decref(nghttp2_rcbuf *rcbuf);
  */
 NGHTTP2_EXTERN nghttp2_vec nghttp2_rcbuf_get_buf(nghttp2_rcbuf *rcbuf);
 
+/**
+ * @function
+ *
+ * Returns 1 if the underlying buffer is statically allocated,
+ * and 0 otherwise. This can be useful for language bindings that wish to avoid
+ * creating duplicate strings for these buffers.
+ */
+NGHTTP2_EXTERN int nghttp2_rcbuf_is_static(const nghttp2_rcbuf *rcbuf);
+
 /**
  * @enum
  *

deps/nghttp2/lib/nghttp2_rcbuf.c

@@ -96,3 +96,7 @@ nghttp2_vec nghttp2_rcbuf_get_buf(nghttp2_rcbuf *rcbuf) {
   nghttp2_vec res = {rcbuf->base, rcbuf->len};
   return res;
 }
+
+int nghttp2_rcbuf_is_static(const nghttp2_rcbuf *rcbuf) {
+  return rcbuf->ref == -1;
+}