framework-pkcs15: Free allocated mechanism
framework-pkcs15: Free allocated mechanism

In the case, when mechanism `mt` is allocated and is directly
assigned to `sign_type`, it needs to be freed at the end of block.
Freeing `mt` does not affect registered mechanism, as the
`sc_pkcs11_register_sign_and_hash_mechanism` works with copy.

Coverity 374844

OpenSC/OpenSC,LudovicRousseau/OpenSC,mouse07410/OpenSC,philipWendland/OpenSC,philipWendland/OpenSC,metsma/OpenSC,mouse07410/OpenSC,LudovicRousseau/OpenSC,metsma/OpenSC,mouse07410/OpenSC,dengert/OpenSC,dengert/OpenSC,OpenSC/OpenSC,OpenSC/OpenSC,LudovicRousseau/OpenSC,dengert/OpenSC,philipWendland/OpenSC,metsma/OpenSC