avcodec/rasc: Fix potential use of uninitialized value

Fixes Coverity issue #1439566.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/rasc.c

@@ -721,6 +721,7 @@ static int decode_frame(AVCodecContext *avctx,
             break;
         default:
             bytestream2_skip(gb, size);
+            ret = 0;
         }
 
         if (ret < 0)