os: fix file read end-of-file detection (#10070)

vlib/os/os_test.v

@@ -160,8 +160,13 @@ fn test_write_and_read_bytes() {
 	// check that trying to read data from EOF doesn't error and returns 0
 	mut a := []byte{len: 5}
 	nread := file_read.read_bytes_into(5, mut a) or {
-		eprintln(err)
-		int(-1)
+		n := if err is none {
+			int(0)
+		} else {
+			eprintln(err)
+			int(-1)
+		}
+		n
 	}
 	assert nread == 0
 	file_read.close()