Deal with Osinfo media == null
Deal with Osinfo media == null

According to my critical logs, "media" can be null.

log_level=G_LOG_LEVEL_CRITICAL, format=
    0x380b2b563a "%s: assertion `%s' failed",
args1=args1@entry=0x7fffffffd2d8) at gmessages.h:101
(log_domain=log_domain@entry=0x380c637e44 "GLib-GObject",
log_level=log_level@entry=
    G_LOG_LEVEL_CRITICAL, format=format@entry=0x380b2b563a "%s:
assertion `%s' failed") at gmessages.c:792
(log_domain=log_domain@entry=0x380c637e44 "GLib-GObject",
    pretty_function=pretty_function@entry=0x380c63b111 "g_object_get",
expression=expression@entry=
    0x380c639f17 "G_IS_OBJECT (object)") at gmessages.c:801
first_property_name=first_property_name@entry=0x45d133 "live")
    at gobject.c:2101
(_data_=0x1b28000) at installer-media.c:563
(simple=0x18d0580 [GSimpleAsyncResult]) at gsimpleasyncresult.c:767
boxes_os_database_guess_os_from_install_media_co (_data_=0x1617360) at
os-database.c:281
at os-database.c:238
(simple=0x198d020 [GSimpleAsyncResult]) at gsimpleasyncresult.c:767

https://bugzilla.gnome.org/show_bug.cgi?id=676589

vbenes/gnome-boxes,vbenes/gnome-boxes