http2: fix null pointer dereference in http2_connisdead

This function can get called on a connection that isn't setup enough to
have the 'recv_underlying' function pointer initialized so it would try
to call the NULL pointer.

Reported-by: Dario Weisser

Follow-up to db1b2c7fe9 (never shipped in a release)
Closes #2536

lib/http2.c

@@ -202,8 +202,11 @@ static bool http2_connisdead(struct connectdata *conn)
          only "protocol frames" */
       CURLcode result;
       struct http_conn *httpc = &conn->proto.httpc;
-      ssize_t nread = ((Curl_recv *)httpc->recv_underlying)(
-        conn, FIRSTSOCKET, httpc->inbuf, H2_BUFSIZE, &result);
+      ssize_t nread = -1;
+      if(httpc->recv_underlying)
+        /* if called "too early", this pointer isn't setup yet! */
+        nread = ((Curl_recv *)httpc->recv_underlying)(
+          conn, FIRSTSOCKET, httpc->inbuf, H2_BUFSIZE, &result);
       if(nread != -1) {
         infof(conn->data,
               "%d bytes stray data read before trying h2 connection\n",