touch: Refuse to set an invalid image or lyrics ID.

touch: Refuse to set an invalid image or lyrics ID.

This makes the callback of the touch command fail the command if the
given ID does not exist in the corresponding blob table.

To this aim we call blob_get_name_by_id() to look up the ID. Since
we are not interested in the name, the function now allows a NULL
result pointer in which case it only checks whether the ID is valid.

With this patch applied the attempt to set an invalid image or lyrics
ID results in an error message like this:

	invalid image ID: 456565
	remote: key not found in rbtree
	main: command failed