git: openjdk/skara: webrev: check that issue linker is not null

[Erik Helin]

Changeset: e4673d67
Author:    Erik Helin <ehelin at openjdk.org>
Date:      2020-02-18 13:48:55 +0000
URL:       https://git.openjdk.java.net/skara/commit/e4673d67

webrev: check that issue linker is not null

Reviewed-by: rwestberg

! webrev/src/main/java/org/openjdk/skara/webrev/Webrev.java