avcodec/svq1dec: Add assert to ensure "stages >= 0"

This is currently always true, the assert protects against
future changes to the code breaking this assumtation
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/svq1dec.c

@@ -193,6 +193,7 @@ static int svq1_decode_block_intra(GetBitContext *bitbuf, uint8_t *pixels,
                     stages, level);
             return AVERROR_INVALIDDATA;  /* invalid vector */
         }
+        av_assert0(stages >= 0);
 
         mean = get_vlc2(bitbuf, svq1_intra_mean.table, 8, 3);
 
@@ -255,6 +256,7 @@ static int svq1_decode_block_non_intra(GetBitContext *bitbuf, uint8_t *pixels,
                     stages, level);
             return AVERROR_INVALIDDATA;  /* invalid vector */
         }
+        av_assert0(stages >= 0);
 
         mean = get_vlc2(bitbuf, svq1_inter_mean.table, 9, 3) - 256;