avcodec/g729postfilter: Fix undefined shifts

Fixes: left shift of negative value -12
Fixes: 17689/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ACELP_KELVIN_fuzzer-5756275014500352

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

libavcodec/g729postfilter.c

@@ -156,7 +156,7 @@ static int16_t long_term_filter(AudioDSPContext *adsp, int pitch_delay_int,
             sig_scaled[i] = residual[i] >> shift;
     else
         for (i = 0; i < subframe_size + RES_PREV_DATA_SIZE; i++)
-            sig_scaled[i] = residual[i] << -shift;
+            sig_scaled[i] = (unsigned)residual[i] << -shift;
 
     /* Start of best delay searching code */
     gain_num = 0;
@@ -500,14 +500,14 @@ static int16_t apply_tilt_comp(int16_t* out, int16_t* res_pst, int refl_coeff,
     tmp = res_pst[subframe_size - 1];
 
     for (i = subframe_size - 1; i >= 1; i--) {
-        tmp2 = (res_pst[i] << 15) + ((gt * res_pst[i-1]) << 1);
-        tmp2 = (tmp2 + 0x4000) >> 15;
+        tmp2 = (gt * res_pst[i-1]) * 2 + 0x4000;
+        tmp2 = res_pst[i] + (tmp2 >> 15);
 
         tmp2 = (tmp2 * ga * 2 + fact) >> sh_fact;
         out[i] = tmp2;
     }
-    tmp2 = (res_pst[0] << 15) + ((gt * ht_prev_data) << 1);
-    tmp2 = (tmp2 + 0x4000) >> 15;
+    tmp2 = (gt * ht_prev_data) * 2 + 0x4000;
+    tmp2 = res_pst[0] + (tmp2 >> 15);
     tmp2 = (tmp2 * ga * 2 + fact) >> sh_fact;
     out[0] = tmp2;