Default the value of CilType.Location.Byte to -1 if missing in JSON

[karoliineh]

While refactoring some GobPie's classes to records, I accidentally removed a seemingly unused field

@SerializedName("byte")
private static int startByte = -1;

from the GoblintLocation class.

This made Goblint crash when using breakpoints for which the requests are done using Location.
Instead of putting the field back into GobPie, we decided to fix it in Goblint, as the of_yojson of a location is anyways only used by GobPie.