Solaris: Do not read from errno when libc did not indicate error (#448)

errno is only guaranteed to be set correctly when the function's return
value indicates that the function failed. Handle the case where an
unexpected negative result is returned separately from the case where
the function failed.

src/solaris.rs

@@ -22,12 +22,11 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
         let ptr = chunk.as_mut_ptr().cast::<c_void>();
         let ret = unsafe { libc::getrandom(ptr, chunk.len(), libc::GRND_RANDOM) };
         // In case the man page has a typo, we also check for negative ret.
-        if ret <= 0 {
-            return Err(last_os_error());
-        }
         // If getrandom(2) succeeds, it should have completely filled chunk.
-        if (ret as usize) != chunk.len() {
-            return Err(Error::UNEXPECTED);
+        match usize::try_from(ret) {
+            Ok(ret) if ret == chunk.len() => {}   // Good. Keep going.
+            Ok(0) => return Err(last_os_error()), // The syscall failed.
+            _ => return Err(Error::UNEXPECTED),
         }
     }
     Ok(())