Fix GError handling

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>

src/config/preferences.c

@@ -89,7 +89,7 @@ _prefs_load(void)
     log_maxsize = g_key_file_get_integer(prefs, PREF_GROUP_LOGGING, "maxsize", &err);
     if (err) {
         log_maxsize = 0;
-        g_error_free(err);
+        PROF_GERROR_FREE(err);
     }
 
     // move pre 0.5.0 autoaway.time to autoaway.awaytime