std.posix.getsockopt() fails to set optlen value

### Zig Version

0.15.0-dev.876+8eca338c2

### Steps to Reproduce and Observed Behavior

```
var fd_type: c_int = 0;
std.posix.getsockopt(fd, std.posix.SOL.SOCKET, std.posix.SO.TYPE, std.mem.asBytes(&fd_type));
// ^ results in EINVAL errno, which is in turn handled as "unreachable" by std.posix code
```

### Expected Behavior

Reachable code that successfully executes `getsockopt()`