SCI: Fix bug #3605269 - "SCI: KQ5 Windows CD - Game Hangs Before Final Battle"

This is caused because songs 1840, 1843 and 1849 are all missing their special
SCI signaling channel (channel 15), so the game scripts wait indefinitely for
signals which are never set
SCI: Set the song's sound queue signal to -1 when stopping a sound, like SSCI does

Previously, we set the signal of the sound objects themselves to -1, but set the
sound queue's signal to 0. This now matches the original behavior.