CLEANUP: stats: shut up a wrong null-deref warning from gcc 9.2

As reported in bug #447, gcc 9.2 invents impossible code paths and then
complains that we don't check for our pointers to be NULL... This code
path is not critical, better add the test to shut it up than try to
help it being less creative.

This code hasn't changed for a while, so it could help distros to
backport this to older releases.

src/stats.c

@@ -2411,7 +2411,7 @@ static void stats_dump_html_head(struct appctx *appctx, struct uri_auth *uri)
 	              "-->\n"
 	              "</style></head>\n",
 	              (appctx->ctx.stats.flags & STAT_SHNODE) ? " on " : "",
-	              (appctx->ctx.stats.flags & STAT_SHNODE) ? (uri->node ? uri->node : global.node) : ""
+	              (appctx->ctx.stats.flags & STAT_SHNODE) ? (uri && uri->node ? uri->node : global.node) : ""
 	              );
 }