BACKPORT: Staging: zram: Fix variable dereferenced before check

(cherry pick from commit 59d3fe540454dd8fc48d4eda44e200f9c98bef10)

This patch fixes the following Smatch warning in zram_drv.c-
drivers/staging/zram/zram_drv.c:899
destroy_device() warn: variable dereferenced before check 'zram->disk' (see line 896)

Bug: 25951511

Change-Id: I4a7e21cee59bb2d3163a010cce62c859d830cd1a
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>