notification: Add an assertion to clarify
notification: Add an assertion to clarify

Coverity doesn't see that g_enum_get_value will never return
NULL here since we always pass it a valid enum value. Help
it along with an assertion.

johne53/MB3Glib,gale320/glib,endlessm/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,johne53/MB3Glib,mzabaluev/glib,ieei/glib,tamaskenez/glib,mzabaluev/glib,tchakabam/glib,Distrotech/glib,ieei/glib,tchakabam/glib,endlessm/glib,cention-sany/glib,Distrotech/glib,endlessm/glib,gale320/glib,johne53/MB3Glib,ieei/glib,johne53/MB3Glib,MathieuDuponchelle/glib,mzabaluev/glib,MathieuDuponchelle/glib,cention-sany/glib,cention-sany/glib,Distrotech/glib,mzabaluev/glib,ieei/glib,tchakabam/glib,tchakabam/glib,gale320/glib,cention-sany/glib,gale320/glib,johne53/MB3Glib,endlessm/glib,cention-sany/glib,tchakabam/glib,tamaskenez/glib,gale320/glib,endlessm/glib,MathieuDuponchelle/glib,mzabaluev/glib,Distrotech/glib,ieei/glib,johne53/MB3Glib,tamaskenez/glib,tamaskenez/glib,Distrotech/glib,tamaskenez/glib