avcodec/g726: Add missing ADDB output mask

Fixes: 1.poc
Fixes out of array read

Found-by: 连一汉 <lianyihan@360.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a5af1240fc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/g726.c

@@ -206,7 +206,7 @@ static int16_t g726_decode(G726Context* c, int I)
 
     if (I_sig)  /* get the sign */
         dq = -dq;
-    re_signal = c->se + dq;
+    re_signal = (int16_t)(c->se + dq);
 
     /* Update second order predictor coefficient A2 and A1 */
     pk0 = (c->sez + dq) ? sgn(c->sez + dq) : 0;