Avoid crash when sound_api function for a key is undefined (#4)

* Avoid crash when sound_api function for a key is undefined

* Refactor function to override sounds using sound_api

* Change override_sounds for validate_sounds

init.lua

@@ -2,9 +2,20 @@ local modpath = minetest.get_modpath("sound_api")
 
 sound_api = dofile(modpath .. "/sound_api_core/init.lua")
 
+local function validate_sound(key)
+    if key and sound_api[key] then
+        return true
+    elseif key then
+        minetest.log("warning", "attempted to call invalid sound: "..key)
+    else
+        minetest.log("warning", "sound_def is missing a sound_api key")
+    end
+    return false
+end
+
 minetest.register_on_mods_loaded(function()
     for name, def in pairs(minetest.registered_nodes) do
-        if def._sound_def and def._sound_def.key then
+        if def._sound_def and validate_sound(def._sound_def.key) then
             minetest.override_item(name, {
                 sounds = sound_api[def._sound_def.key](def._sound_def.input)
             })
@@ -13,7 +24,7 @@ minetest.register_on_mods_loaded(function()
 
     local old_reg_node = minetest.register_node
     function minetest.register_node(name, def)
-        if def._sound_def and def._sound_def.key then
+        if def._sound_def and validate_sound(def._sound_def.key) then
             def.sounds = sound_api[def._sound_def.key](def._sound_def.input)
         end