HevTunnelMacOS: Fix signed/unsigned comparison in read/write result checks (#286)

Cast `sizeof(type)` to `ssize_t` when comparing against the return values of `read` and `write`, avoiding signed/unsigned mismatches and ensuring correct error handling on macOS.

Author: yujinpan

src/hev-tunnel-macos.h

@@ -30,7 +30,7 @@ hev_tunnel_read (int fd, int mtu, HevTaskIOYielder yielder, void *yielder_data)
     iov[1].iov_len = buf->len;
 
     s = hev_task_io_readv (fd, iov, 2, yielder, yielder_data);
-    if (s <= sizeof (type)) {
+    if (s <= (ssize_t)sizeof (type)) {
         pbuf_free (buf);
         return NULL;
     }
@@ -67,7 +67,7 @@ hev_tunnel_write (int fd, struct pbuf *buf)
     }
 
     res = writev (fd, iov, i);
-    if (res <= sizeof (type))
+    if (res <= (ssize_t)sizeof (type))
         return -1;
 
     return res;