Several users have reported segfaults when starting up profanity which
has OMEMO support, but OMEMO is not set up yet.
@StefanKropp has been able to reproduce this and tracked it down to
`_load_identity()` calling `omemo_known_devices_keyfile_save()`.
The latter then calls `save_keyfile()` which calls
`g_key_file_save_to_file()`. This can then fail if one of the first two
strings is NULL and won't set the `error` on return. In its error handling
`save_keyfile()` unconditionally dereferences `error` which leads to the
segfault.
Fix this and also go through the entire codebase and verify that the usage
of `GError` is done correctly.
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
48ac88de08