Fix invalid st0 use in logs

[fengjian]

warning: variable `st0` is assigned to, but never used
   --> src/library/lib.rs:327:17
    |
327 |         let mut st0 = registers.st0;
    |                 ^^^
    |
    = note: consider using `_st0` instead
    = note: `#[warn(unused_variables)]` on by default

warning: value assigned to `st0` is never read
   --> src/library/lib.rs:356:13
    |
356 |             st0 = registers.st0;
    |             ^^^
    |
    = help: maybe it is overwritten before being read?
    = note: `#[warn(unused_assignments)]` on by default

[dr-orlovsky]

ACK 17b340b

[dr-orlovsky]

Welcome and thank you for finding & fixing!