Use i32::unsigned_abs() instead of (-x) as u32.

(-x) overflows when x is the minimum value, where `x.unsigned_abs()`
does the right thing.

src/hermit.rs

@@ -2,11 +2,6 @@
 use crate::Error;
 use core::{mem::MaybeUninit, num::NonZeroU32};
 
-/// Minimum return value which we should get from syscalls in practice,
-/// because Hermit uses positive `i32`s for error codes:
-/// https://github.com/hermitcore/libhermit-rs/blob/main/src/errno.rs
-const MIN_RET_CODE: isize = -(i32::MAX as isize);
-
 extern "C" {
     fn sys_read_entropy(buffer: *mut u8, length: usize, flags: u32) -> isize;
 }
@@ -18,9 +13,13 @@ pub fn getrandom_inner(mut dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
         if res > 0 && (res as usize) <= dest.len() {
             dest = &mut dest[res as usize..];
         } else {
-            let err = match res {
-                MIN_RET_CODE..=-1 => NonZeroU32::new(-res as u32).unwrap().into(),
-                _ => Error::UNEXPECTED,
+            let err = if res < 0 {
+                u32::try_from(res.unsigned_abs())
+                    .ok()
+                    .and_then(NonZeroU32::new)
+                    .map_or(Error::UNEXPECTED, Error::from)
+            } else {
+                Error::UNEXPECTED
             };
             return Err(err);
         }

src/solid.rs

@@ -13,6 +13,6 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     } else {
         // ITRON error numbers are always negative, so we negate it so that it
         // falls in the dedicated OS error range (1..INTERNAL_START).
-        Err(NonZeroU32::new((-ret) as u32).unwrap().into())
+        Err(NonZeroU32::new(ret.unsigned_abs()).unwrap().into())
     }
 }