Receiving error in local variable err, and forgot to free it.
This patch check the return value of 'gdk_window_create_gl_context'
and 'gdk_gl_context_realize', then free err to fix it.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Message-Id: <20200831134315.1221-6-pannengyuan@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>