hevc_mvs: set candidate availabilities

They might be left uninitialized otherwise since 3ad04608.

Fixes ticket #3840.

Found-by: Carl Eugen Hoyos <ce@hoyos.ws>
Reported-by: Piotr Bandurski <ami_stuff@o2.pl>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
