This fixes ticket #7997 as well as the vsynth*-prores_# FATE-tests
(where * ranges over { 1, 2, 3, _lena } and # over { , _int, _444,
_444_int }).

(Given that prev_dc is in the range -0xC000..0x3FFF, no overflow can
happen upon multiplication with 2.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>