bstr: check for overflow in buffer allocation

We're being a little bit lazy here and limit the max allocation to
SIZE_MAX/2, which is practically infinite anyway on 64 bit systems.
audio: check for overflows
stream_pvr: fix crash on initialization

Fallout from the mp_msg conversions.
dvdnav: set correct stride for fake highlights

This didn't really matter, because the fake highlight rectangle is in a
single color, and every pixel has the same color value.