Ensure we call G_IS_ENUM_CLASS() or G_IS_FLAGS_CLASS() only on classed
types.

 $ nmcli connection modify foobar wifi.powersave 1
 Thread 1 "nmcli" received signal SIGSEGV, Segmentation fault.
 0x0000000000440a4d in _set_fcn_gobject_enum at
 clients/common/nm-meta-setting-desc.c:985
 985                 || G_IS_ENUM_CLASS (gtype_prop)) {

Fixes: f53218ed