windows: fix uninitialized local variable warning
windows: fix uninitialized local variable warning

Pass the user-returned buffer to the read calback. Fixes the following
warning:

    src\win\tcp.c(439): warning C4700: uninitialized local variable
    'buf'

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso