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'
windows: fix missing return value warning

Fixes the following warning:

    src\uv-common.c(476): warning C4715:
    'uv__getaddrinfo_translate_error' : not all control paths return
    a value

The function never returns - the final statement is a call to abort() -
but it seems MSVC's program flow analyzer is too weak to figure that
out.