Fix seekEOCDR: don't search entire file.

For a large file missing EOCDR, zip.js can freeze or even crash browser.
Thanks for the explanation.

To make the code more easy to follow, could you reduce the number of variables to 2 ? The read offset and the buffer size (which should almost always have a fixed value, except possibly for the last iteration, when there are not enough bytes to fill the buffer).

Copy-paste (and initialize `bufSize = Math.min(1024, reader.size - offset);`:

```javascript
if (offset === 0) {
    onerror(ERR_BAD_FORMAT); // EOCDR not found
    return;
}
// Advance the offset. Include part of the current buffer (EOCDR_MIN + 1)
// to make sure that EOCDR that exceeds the buffer boundaries is still found.
offset -= bufSize - EOCDR_MIN + 1;
if (offset < 0) {
    bufSize += offset; // Adjust buffer
    offset = 0;
}
step();
```