lib/relocator: Fix dereference after NULL check

lib/relocator: Fix dereference after NULL check

In the function free_subchunk(), after checking that subchu->post isn't NULL,
grub_memset() is called on subchu->pre->freebytes but it should be called on
subchu->post->freebytes. If subchu->pre is NULL but subchu->post isn't NULL,
then this could lead to a NULL pointer dereference.

Fixes: CID 473882
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>