Fix seekEOCDR: don't search entire file.

For a large file missing EOCDR, zip.js can freeze or even crash browser.
Sorry for this mess. This code firstly search last 18-21 bytes for EOCDR; if it is not found, read & search last 22-43 bytes; if it is not found, read & search last 44-87 bytes, and so on, until EOCDR_MAX -- each time enlarge the seach length by factor of 2. The purpose was making the searching fast for zip files without or with short comments. But it seems not worth the complexity.

I think I can simplify the algorithm to 2 steps: firstly read last 22 bytes and search last 18-21 bytes; if it is not found, read last 64K + 22 bytes and search reversely. This should keep the searching for zip files without comment equally fast. What do you think?