fix memory leaks when using g_hashtable

fix memory leaks when using g_hashtable

when generating a bitmap for a character whose codepoint was above 255,
we used a cache_id of 0, malloc'd a CachedImage and inserted it into a
g_hashtable, without freeing the one which was before inserted with cache_id 0

this is circumvented by only generating a CachedImage when having
a cache_id != 0

the second leak was also with inserting into a hashtable, but there we
give the hashtable the g_free method as a value_destroy_func

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>