Fix uninitialized variable in Windows get_file_posn.

Fix uninitialized variable in Windows get_file_posn.

The Windows implementation of get_file_posn is calling SetFilePointer
to obtain the current position in the file. However it did not
initialize the variable holding the high order 32-bit to 0. Thus,
SetFilePointer either returned -1 to indicate an error or did move the
file pointer to a different location instead of just returning the
current position. This change just initializes the variable to 0.

As a result, this bug has caused psftp's reget command to fail
resuming transfers or to create corrupt files due to setting up an
incorrect resume offset.