lavfi/perlin: Fix out of bounds stack buffer write

An incorrect calculation in ff_perlin_init causes a write to the
stack array at index 256, which is out of bounds.

Fixes: CID1608711
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/perlin.c

@@ -129,7 +129,7 @@ int ff_perlin_init(FFPerlin *perlin, double period, int octaves, double persiste
         for (i = 0; i < 256; i++) {
             unsigned int random_idx = av_lfg_get(&lfg) % (256-i);
             uint8_t random_val = random_permutations[random_idx];
-            random_permutations[random_idx] = random_permutations[256-i];
+            random_permutations[random_idx] = random_permutations[255-i];
 
             perlin->permutations[i] = perlin->permutations[i+256] = random_val;
         }