Fixes libwinpr build issue on Windows

Adds conditional statements in file.c to solve a Posix portability
issue on Windows introduced with the following commit:

913d532e0d3456945236567af00e8119564307ff
codec/rfx: fix multithreaded encoder

Some component of the encoder chain (I suspect the rlgr encoder) expects
the output buffer to be zeroed. The multithreaded RemoteFX encoder uses
wStreams from the StreamPool which are reused and not zeroed out of
course. For now, in order to prevent data corruption we clear the stream.