armv7m: Improve armv7m_blank_check_memory() error handling

Clean up the working area in case writing fails.
Adapted from armv7m_checksum_memory().

Change-Id: I4e5950f568ed70a72a1dcfd77e3321110b17e1de
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3469


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>