Fix seekEOCDR: don't search entire file.

For a large file missing EOCDR, zip.js can freeze or even crash browser.
The buffers do overlap (maybe my comment above is not byte-accurate). In 1st round, begin at line 599, 
```
offset = reader.size - EOCDR_MIN
dist = EOCDR_MIN
newOffset = reader.size - EOCDR_MIN * 2
searchLength = EOCDR_MIN
offset = reader.size - EOCDR_MIN * 2
bufSize = EOCDR_MIN * 2 - 1
```
In 2nd round, search begins at `searchLength - 1` and 3 bytes afterward in the buffer, that is, `reader.size - EOCDR_MIN - 1` and 3 bytes afterward in the file. Note that 1st round search begins at `reader.size - EOCDR_MIN` and 3 bytes afterward in the file, so there is a overlap. I appended 1 byte to a normal zip file, and this algorithm did find the EOCDR at the 2nd round.

Also `bufSize` is always bigger than `searchLength` by `EOCDR_MIN - 1`, so won't read out of bound.

Also the `bufSize` is increase by 2 each time, so it won't take too many rounds to hit EOCDR_MAX.