swscale/utils: Allocate more dithererror

Fixes: out of array read
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/utils.c

@@ -1885,7 +1885,7 @@ static av_cold int sws_init_single_context(SwsContext *c, SwsFilter *srcFilter,
     }
 
     for (i = 0; i < 4; i++)
-        if (!FF_ALLOCZ_TYPED_ARRAY(c->dither_error[i], c->dstW + 2))
+        if (!FF_ALLOCZ_TYPED_ARRAY(c->dither_error[i], c->dstW + 3))
             goto nomem;
 
     c->needAlpha = (CONFIG_SWSCALE_ALPHA && isALPHA(c->srcFormat) && isALPHA(c->dstFormat)) ? 1 : 0;