An ambiguous bang like `!!d` raises an exception in function get_bang_url(). A
bang is only unique when the bang_definition from get_bang_definition_and_ac() is
a string / for a ambiguous bang the returned bang_definition is a dictionary.
Reported-by: user prg at #searxng:matrix.org on 2022/01/11
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>