Fix: flt.utils.muxer: make sure message's default clock class exists

Without the checks added by this patch, the iterator can borrow the
default clock snapshot of a message related to a stream class without
any default clock class. This is a precondition break.

If there's no default clock class, then we fall back to assuming this
message has the same time as the latest returned message's time.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>