Fix the sfx selection gui for playsound (#10095)

* fix the index used to get the sfx substring

* handle cases where the sxf variable is not a built-in

This prevents a crash that happened whenever the sfx
variable's name had less than 5 characters (4 before this
pr), since the substring method was called regardless of
the name's size.

core/src/mindustry/logic/LStatements.java

@@ -2149,7 +2149,8 @@ public class LStatements{
             table.button(b -> {
                 b.image(Icon.pencilSmall);
                 
-                b.clicked(() -> showSelect(b, GlobalVars.soundNames.toArray(String.class), id.substring(4), t -> {
+                String soundName = id.startsWith("@sfx-") ? id.substring(5) : id;
+                b.clicked(() -> showSelect(b, GlobalVars.soundNames.toArray(String.class), soundName, t -> {
                     id = "@sfx-" + t;
                     rebuild(table);
                 }, 2, cell -> cell.size(160, 50)));