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/main.c

@@ -93,9 +93,9 @@ main(int argc, char** argv)
     context = g_option_context_new(NULL);
     g_option_context_add_main_entries(context, entries, NULL);
     if (!g_option_context_parse(context, &argc, &argv, &error)) {
-        g_print("%s\n", error->message);
+        g_print("%s\n", PROF_GERROR_MESSAGE(error));
         g_option_context_free(context);
-        g_error_free(error);
+        PROF_GERROR_FREE(error);
         return 1;
     }