[BusyBox] tar segfaults

[Jonas Holmberg]

when extracting a regular file if BUFSIZ < 512. I noticed when I was using an
older uC-libc with BUFSIZ 500. (BB 0.52)

It's easy to fix: In the function tarExtractRegularFile, either change the two 
occurences of 512 (used for mod operation) to BUFSIZ or change the size of the
char[] buffer to 512 (or maybe it's supposed to be TAR_BLOCK_SIZE, which is 512).

/Jonas