avformat/aviobuf: zero initialize the AVIOContext in ffio_init_context()

This makes sure no field is ever used uninitialized.

Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: James Almer <jamrial@gmail.com>