Fix strerror_r error test

`man 3 strerror_r` says:

The XSI-compliant strerror_r() function returns 0 on success. On
error, a (positive) error number is returned (since glibc 2.13), or -1
is returned and errno is set to indicate the error (glibc versions
before 2.13).

src/unix.rs

@@ -30,7 +30,7 @@ where
 {
     let mut buf = [0u8; 1024];
     let c_str = unsafe {
-        if strerror_r(err.0, buf.as_mut_ptr() as *mut _, buf.len() as size_t) < 0 {
+        if strerror_r(err.0, buf.as_mut_ptr() as *mut _, buf.len() as size_t) != 0 {
             let fm_err = errno();
             if fm_err != Errno(libc::ERANGE) {
                 return callback(Err(fm_err));