grilo: fix crash due NULL url from GrlMedia

 #0 g_str_hash (v=0x0) at ghash.c:1876
 #1 in g_hash_table_lookup_node (hash_return=<synthetic pointer>,
    key=0x0, hash_table=0x604640) at ghash.c:375
 #2 g_hash_table_lookup (hash_table=0x604640, key=key@entry=0x0) at ghash.c:1147
 #3 in rb_refstring_find (init=0x0) at rb-refstring.c:122
 #4 in rhythmdb_entry_lookup_by_location (db=0x76a490, uri=<optimized out>)
    at rhythmdb.c:4036
 #5 in create_entry_for_media (container=0x0, data=0xcb5360,
    entry_type=<optimized out>, db=0x76a490) at rb-grilo-source.c:553

https://bugzilla.gnome.org/show_bug.cgi?id=766392