1) Proactively set placeholder sounds to invalid so that SEXPs or scripts that may reference invalid sounds won't cause a crash.
2) Ensure that named sounds, whose indexes do not matter, are filled into the placeholder sound slots. This prevents sounds from being appended into unnecessary extra indexes. Sounds are now filled in as they were before, while retaining the previous fix of not overwriting reserved slots.
1) Proactively set placeholder sounds to invalid so that SEXPs or scripts that may reference invalid sounds won't cause a crash. 2) Ensure that named sounds, whose indexes do not matter, are filled into the placeholder sound slots. This prevents sounds from being appended into unnecessary extra indexes. Sounds are now filled in as they were before, while retaining the previous fix of not overwriting reserved slots.
Follow-up to #6112.