Coverity says
CID 202453 (#1 of 1): Wrong sizeof argument (SIZEOF_MISMATCH)suspicious_sizeof:
Passing argument user_data of type gconstpointer and argument (gsize)nargs * 8UL /* sizeof (gconstpointer) */ to function g_slice_free1 is suspicious.
Let's pass instead the "data" pointer. It's the same, but maybe that
avoids the warning.