[Lua] mapmusname isn't being overwritten properly

No terminator character is copied into the field, thus making parts of a previous name seek through if it is longer than the one it's being set to.

Try it with this script: pressing Custom 1 will set mapmusname to "AGZALT" and Custom 2 will set it to "THZ2". In both cases, P_RestoreMusic is called afterwards to play the music currently stored in mapmusname, and its current value is printed into the console.

Pressing Custom 2 after having pressed Custom 1 will instead report that mapmusname was set to "THZ2LT" - the LT part was carried over from "AGZALT".

This was discussed on the Discord server as well.