avcodec/g729postfilter: Fix undefined intermediate pointers

Fixes: index -49 out of bounds for type 'int16_t [192]'
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>
(cherry picked from commit 0c61661a2c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/g729postfilter.c

@@ -201,8 +201,8 @@ static int16_t long_term_filter(AudioDSPContext *adsp, int pitch_delay_int,
         }
         if (corr_int_num) {
             /* Compute denominator of pseudo-normalized correlation R'(0). */
-            corr_int_den = adsp->scalarproduct_int16(sig_scaled - best_delay_int + RES_PREV_DATA_SIZE,
-                                                    sig_scaled - best_delay_int + RES_PREV_DATA_SIZE,
+            corr_int_den = adsp->scalarproduct_int16(sig_scaled + RES_PREV_DATA_SIZE - best_delay_int,
+                                                     sig_scaled + RES_PREV_DATA_SIZE - best_delay_int,
                                                     subframe_size);
 
             /* Compute signals with non-integer delay k (with 1/8 precision),