Normalize VAR section before outputting ALLOC in decompiled file

[x87]

Some old compiled script may have suffered from #223 and have negative indexes stored in SB Footer's VAR section.

After #251 this results in lines like this:

Alloc($var, -1000)

Also some variable names could be malformed and contain invalid characters (e.g. // ). Need to investigate whether it is still possible for CustomVariables.ini to have such names.

Alloc($var//[0], -1000)