Fix ttvdb.py to get coverarts for seasons.

Fetching the coverart with the scrip ttvdb.py fails,
if the artwork is listed in data['_banners']['season'],
but the section data['_banners']['poster'] is missing.
In this case, the xslt transformation does not work as expected.

Checked with ttvdb.py -l de -a CH -D 89901 36 4

mythtv/programs/scripts/metadata/Television/ttvdb.py

@@ -2068,6 +2068,13 @@ def series_images_item_func(parent):
         return "Banner"
     for show_id in t.shows.keys():
         break
+
+    # dict for 'data['_banners']['poster']['raw'] must exist for fetching coverarts,
+    # check with ttvdb.py -l de -a CH -D 89901 36 4
+    if 'poster' not in t.shows[show_id].data['_banners'].keys():
+        t.shows[show_id].data['_banners']['poster'] = {}
+        t.shows[show_id].data['_banners']['poster']['raw'] = {}
+
     # sort the cast into sort order
     t.shows[show_id].data['_actors'] = sorted(t.shows[show_id].data['_actors'], key=lambda k: k['sortOrder'])
     t.searchTree = None