avcodec/g723_1dec: Fix runtime error: left shift of negative value -1

Fixes: 1504/clusterfuzz-testcase-minimized-6249212138225664

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/g723_1dec.c

@@ -695,13 +695,13 @@ static int estimate_sid_gain(G723_1_Context *p)
     if (y <= 0) {
         t = seg * 32 + (val + 1 << seg2);
         t = t * t - x;
-        val = (seg2 - 1 << 4) + val;
+        val = (seg2 - 1) * 16 + val;
         if (t >= y)
             val++;
     } else {
         t = seg * 32 + (val - 1 << seg2);
         t = t * t - x;
-        val = (seg2 - 1 << 4) + val;
+        val = (seg2 - 1) * 16 + val;
         if (t >= y)
             val--;
     }