pstore: Set tfm to NULL on free_buf_for_compression
Set tfm to NULL on free_buf_for_compression() after crypto_free_comp().
This avoid a use-after-free when allocate_buf_for_compression()
and free_buf_for_compression() are called twice. Although
free_buf_for_compression() freed the tfm, allocate_buf_for_compression()
won't reinitialize the tfm since the tfm pointer is not NULL.