avcodec/av1dec: Fix resolving zero divisor

Fixes: Out of array read
Fixes: global-buffer-overflow-AV1

Found-by: "Leonelli, Matteo" <matteo.leonelli@cispa.de>
Tested-by: "Wang, Fei W" <fei.w.wang@intel.com>
Reviewed-by: "Wang, Fei W" <fei.w.wang@intel.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/av1dec.c

@@ -177,7 +177,7 @@ static uint8_t get_shear_params_valid(AV1DecContext *s, int idx)
     int16_t alpha, beta, gamma, delta, divf, divs;
     int64_t v, w;
     int32_t *param = &s->cur_frame.gm_params[idx][0];
-    if (param[2] < 0)
+    if (param[2] <= 0)
         return 0;
 
     alpha = av_clip_int16(param[2] - (1 << AV1_WARPEDMODEL_PREC_BITS));