avcodec/vp3: Error out on invalid num_coeffs in unpack_vlcs()

This fixes a hypothetical integer overflow
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp3.c

@@ -951,9 +951,11 @@ static int unpack_vlcs(Vp3DecodeContext *s, GetBitContext *gb,
     Vp3Fragment *all_fragments = s->all_fragments;
     VLC_TYPE(*vlc_table)[2] = table->table;
 
-    if (num_coeffs < 0)
+    if (num_coeffs < 0) {
         av_log(s->avctx, AV_LOG_ERROR,
                "Invalid number of coefficients at level %d\n", coeff_index);
+        return AVERROR_INVALIDDATA;
+    }
 
     if (eob_run > num_coeffs) {
         coeff_i      =