erofs: fix double free of 'copied'

Dan reported a new smatch warning [1]
"fs/erofs/inode.c:210 erofs_read_inode() error: double free of 'copied'"

Due to new chunk-based format handling logic, the error path can be
called after kfree(copied).

Set "copied = NULL" after kfree(copied) to fix this.

[1] https://lore.kernel.org/r/202108251030.bELQozR7-lkp@intel.com

Link: https://lore.kernel.org/r/20210825120757.11034-1-hsiangkao@linux.alibaba.com
Fixes: c5aa903a

 ("erofs: support reading chunk-based uncompressed files")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>