some compilation errors on platformio

[speriator]

my platformio ask me some modifications like this in cpu.c (missing break;) to compile correctly.

switch (reg_code) {
    case 0x01:
        reg_ptr = &reg.y;
        break;
    case 0x02:
        reg_ptr = &reg.s;
        break;
    case 0x03:
        reg_ptr = &reg.u;
        break;
}

and a return of something at the end of process_next_instruction()

uint32_t process_next_instruction() {
// See Zaks p.250

...

return 0;

}