avcodec/flashsv2enc: Fix undefined NULL + 0

Affected the vsynth*-flashsv2 FATE-tests.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/flashsv2enc.c

@@ -159,7 +159,7 @@ static void init_blocks(FlashSV2Context * s, Block * blocks,
             b->enc   = encbuf;
             b->data  = databuf;
             encbuf  += b->width * b->height * 3;
-            databuf += !databuf ? 0 : b->width * b->height * 6;
+            databuf  = databuf ? databuf + b->width * b->height * 6 : NULL;
         }
     }
 }