avcodec/wavarc: Check that nb_samples is not negative

It is currently probably not possible for it to be negative as
the needed 2Mb input buf size is not achievable. But it is more
robust to check for it too.
If it would become negative than code like
s->samples[0][n] = s->samples[0][s->nb_samples + n];
would crash

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wavarc.c

@@ -311,7 +311,7 @@ static int decode_2slp(AVCodecContext *avctx,
             return AVERROR_EOF;
         case 8:
             s->nb_samples = get_urice(gb, 8);
-            if (s->nb_samples > 570) {
+            if (s->nb_samples > 570U) {
                 s->nb_samples = 570;
                 return AVERROR_INVALIDDATA;
             }
@@ -587,7 +587,7 @@ static int decode_5elp(AVCodecContext *avctx,
             return AVERROR_EOF;
         case 11:
             s->nb_samples = get_urice(gb, 8);
-            if (s->nb_samples > 570) {
+            if (s->nb_samples > 570U) {
                 s->nb_samples = 570;
                 return AVERROR_INVALIDDATA;
             }