Fix SFTPAttribute unpacking.

SFTP running on Cygwin may return 0xffffffff for unknown uid and gid which creates a problem when paramiko tries to interpret it as a big_int. The SFTP draft [1] specifies the 'File Attributes' fields to be int32, so the SFTPAttribute unpacking code must use get_size() after the change by scottkmaxwell to support big_int [2].

[1] http://tools.ietf.org/id/draft-ietf-secsh-filexfer-13.txt
[2] https://github.com/paramiko/paramiko/commit/0e4ce3762a5b25c5d3eb89335495d3bb9054e3e7