change buffer length types from int to off_t

previously the length of the buffer in the peek reader was stored in an int. when
a large read (> INT32_MAX) was requested, the buffer-resize malloc (ior-peek.c:129)
would fail, and therefore trigger the assert on line 135.

this bug was triggered, and the fix tested, on FreeBSD 10.
Merge branch 'fix_wandio_peek_large_read' of https://github.com/alistairking/libtrace into develop