protect against jAccContext being NULL
protect against jAccContext being NULL

As seen with netbeans (see https://bugs.debian.org/798273), jAccContext may
be NULL (getAccessibleContext() is allowed to return null), so we need to
protect the ATK wrapper against this.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=754688

GNOME/java-atk-wrapper,GNOME/java-atk-wrapper,GNOME/java-atk-wrapper