shared: try avoid coverity warning in _nm_utils_user_data_unpack()

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.

shared/nm-glib-aux/nm-shared-utils.c

@@ -2878,7 +2878,7 @@ _nm_utils_user_data_unpack (gpointer user_data, int nargs, ...)
 	}
 	va_end (ap);
 
-	g_slice_free1 (((gsize) nargs) * sizeof (gconstpointer), user_data);
+	g_slice_free1 (((gsize) nargs) * sizeof (gconstpointer), data);
 }
 
 /*****************************************************************************/