fsck.f2fs: nullify raw_super pointer
fsck.f2fs: nullify raw_super pointer

Otherwise it will cause double free.

Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

TeamExodus/external_f2fs-tools,TeamExodus/external_f2fs-tools