Crashing bug with some SPCs

[Former user]

Certain SPCs, for example Final Fantasy V/1.12 - Good Night!.spc (attached), cause GME to crash with SIGFPE. This was experienced and debugged using a debug build of Cog circa version 1920.

[Christopher Snowhill]

Possibly a division by zero error with the fadeout field being 0. I had to deal with that bug back when I was using my previous fork of GME.

[Christopher Snowhill]

Needs a check at the start of Music_Emu::handle_fade() for zero fade_step, since int_log() divides by that value, which can throw a division by zero error. Strangely, it did not occur on AArch64, only Intel.

[mywave]

Does not seem to be a problem with current git master on x86_64 Linux

[Wohlstand]

I tested the thing, and seems, it works just fine.