secvar: Free md context on hash error
secvar: Free md context on hash error

There were a few instances in `get_hash_to_verify` where NULL is
returned before unallocating the md context. This commit ensures that
this memory is properly freed before returning.

Signed-off-by: Nick Child <47354f8ad04d636edc050e4b8e3c4d452258c667@ibm.com>
Signed-off-by: Vasant Hegde <a6d753843573f7bfed5075c1e875b0be8007a0ab@linux.vnet.ibm.com>

open-power/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,qemu/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot