Fix #7 - don't ignore errors from pread(2)

Use a signed type for result from pread(2), so the error handling case
actually trips. Good thing pread(2) should basically never error for
local files =).

CR: gcc, tests (with py.test 2.2.4, if it matters)
Merge pull request #8 from cemeyer/fix_7_fix_pread_error_handling

Fix #7 - don't ignore errors from pread(2)