core: Initialize key return value in gpgme_get_key
* src/keylist.c (gpgme_get_key): Set r_key to NULL.
The c++ bindings and others assumed that r_key is set to NULL
on error. This is the behavior gpgme_op_keylist_next also
has. Even if it is not specified what happens to r_key on
error setting it to NULL should not hurt and is more
This directly fixes an uninitialized memory access error
in the c++ bindings / Kleopatra:
And will fix some additional random crashes in Kleopatra and GpgOL.
- GnuPG-Bug-Id: T3865