Initialize m_readbuf in ctor

In commit e2ec92f129f32c42442cf5c46ef92da0e7c90d20
m_readbuf was initialized in mythsystemunix.cpp
based on cppcheck warning.  Do the same in
mythsystemwindows.cpp for consistency (and because
if static analysis was run on the windows source,
it would likely report the same issue).

Signed-off-by: Gary Buhrmaster <gary.buhrmaster@gmail.com>