sounds: squelch NPC messages regarding hearing footsteps

[mlangsdorf]

Summary

SUMMARY: Bugfixes "sounds: squelch NPC messages regarding hearing footsteps"

Purpose of change

Players with NPCs nearby are hearing footsteps constantly. The footsteps are coming from the NPCs processing ambient sounds into the player-only section of sound processing, and displaying "add_msg" messages that were really intended to be player only.

Remove the errant sound markers and messages.

Describe the solution

Change the NPC cut-off logic to be a two-stage test:

if( npc ) {
    if( !sound.ambient ) {
        npc->hear_sound( sound );
    }
    continue;
}

instead of the current single stage test

if( npc && !sound.ambient ) {
    npc->hear_sound( sound );
    continue;
}

which continues sound processing for when NPCs hear ambient sounds.