avcodec/g729dec: Avoid computing invalid temporary pointers for ff_acelp_weighted_vector_sum()

Fixes: Ticket8176
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/g729dec.c

@@ -557,12 +557,13 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame_ptr,
               fc_v[i] = <
                          \ fc_v[i] + gain_pitch * fc_v[i-pitch_delay], i >= pitch_delay
             */
-            ff_acelp_weighted_vector_sum(fc + pitch_delay_int[i],
-                                         fc + pitch_delay_int[i],
-                                         fc, 1 << 14,
-                                         av_clip(ctx->past_gain_pitch[0], SHARP_MIN, SHARP_MAX),
-                                         0, 14,
-                                         SUBFRAME_SIZE - pitch_delay_int[i]);
+            if (SUBFRAME_SIZE > pitch_delay_int[i])
+                ff_acelp_weighted_vector_sum(fc + pitch_delay_int[i],
+                                             fc + pitch_delay_int[i],
+                                             fc, 1 << 14,
+                                             av_clip(ctx->past_gain_pitch[0], SHARP_MIN, SHARP_MAX),
+                                             0, 14,
+                                             SUBFRAME_SIZE - pitch_delay_int[i]);
 
             memmove(ctx->past_gain_pitch+1, ctx->past_gain_pitch, 5 * sizeof(int16_t));
             ctx->past_gain_code[1] = ctx->past_gain_code[0];