avcodec/sonic: Use unsigned temporary in predictor_calc_error()

Fixes: signed integer overflow: -2147471366 - 18638 cannot be represented in type 'int'
Fixes: 30157/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SONIC_fuzzer-5171199746506752

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

libavcodec/sonic.c

@@ -475,13 +475,13 @@ static int predictor_calc_error(int *k, int *state, int order, int error)
     for (i = order-2; i >= 0; i--, k_ptr--, state_ptr--)
     {
         int k_value = *k_ptr, state_value = *state_ptr;
-        x -= shift_down(k_value * (unsigned)state_value, LATTICE_SHIFT);
+        x -= (unsigned)shift_down(k_value * (unsigned)state_value, LATTICE_SHIFT);
         state_ptr[1] = state_value + shift_down(k_value * (unsigned)x, LATTICE_SHIFT);
     }
 #else
     for (i = order-2; i >= 0; i--)
     {
-        x -= shift_down(k[i] * state[i], LATTICE_SHIFT);
+        x -= (unsigned)shift_down(k[i] * state[i], LATTICE_SHIFT);
         state[i+1] = state[i] + shift_down(k[i] * x, LATTICE_SHIFT);
     }
 #endif