avfilter/vf_hqdn3d: Fix left-shift of negative numbers

Affected filter-hqdn3d and filter-hqdn3d-sample FATE-tests.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_hqdn3d.c

@@ -180,7 +180,7 @@ static void precalc_coefs(double dist25, int depth, int16_t *ct)
     gamma = log(0.25) / log(1.0 - FFMIN(dist25,252.0)/255.0 - 0.00001);
 
     for (i = -(256<<LUT_BITS); i < 256<<LUT_BITS; i++) {
-        double f = ((i<<(9-LUT_BITS)) + (1<<(8-LUT_BITS)) - 1) / 512.0; // midpoint of the bin
+        double f = (i * (1 << (9-LUT_BITS)) + (1<<(8-LUT_BITS)) - 1) / 512.0; // midpoint of the bin
         simil = FFMAX(0, 1.0 - fabs(f) / 255.0);
         C = pow(simil, gamma) * 256.0 * f;
         ct[(256<<LUT_BITS)+i] = lrint(C);