Fix seekEOCDR: don't search entire file.

For a large file missing EOCDR, zip.js can freeze or even crash browser.
Change `doSeek(...)` to
```
doSeek(EOCDR_MIN, function() {
    doSeek(Math.min(EOCDR_MAX, reader.size), function() {
        onerror(ERR_BAD_FORMAT);
    });
});
```