persistent_ram: check PERSISTENT_RAM_SIG before writing

if the PERSISTENT_RAM_SIG was corrupted, do not write something
to the persistent_ram buffer. this may cause the abnormal crash.

author Jaeseong GIM <jaeseong.gim@lge.com> Sun Aug 05 00:15:12 2012 -0700
committer Jaeseong GIM <jaeseong.gim@lge.com> Mon Aug 06 11:27:41 2012 -0700