gpg: release DEK soon after its use.
* g10/keygen.c (generate_subkeypair): Release DEK soon.
This fixes the out_of_core error in the test case of adding
RSA-4096 subkey to RSA-4096 primary key with configuration:
s2k-cipher-algo S10
Cherry-picked da66ad5bba4215b9ddd0cb927a89aa75355632aa from
STABLE-BRANCH-1-4 branch.
- Debian-bug-id: #772780