Core dump on track remove

[DenisBY]

Steps:

1. Add track
2. Shift+Ctrl+X

Result:

$ zytrax 
one
two
three
 *** ERROR *** globals/json_file.cpp:26 - !f failed.
regfx
 *** ERROR *** globals/json_file.cpp:26 - !f failed.
go
 *** ERROR *** engine/song.cpp:373 - index out of size: 0(0)
Segmentation fault (core dumped)

It doesn't matter if there is only one track or several.