Fix sftp_get_file returning success on failed transfers.

Fix sftp_get_file returning success on failed transfers.

Due to the return variable 'ret' being shadowed in the transfer loop, errors
in the transfer loop did not affect the final return value of sftp_get_file.
This particularly affects psftp's batch mode (without passing the -be
command-line argument), which would errorneously continue. The solution is
to simply remove the shadowing declaration.