avcodec/g729postfilter: Clip gain before scaling with AGC_FAC1

The fixed point integer reference specifies the multiplication used
to have 16bit input and clips so we need to clip the input
The floating point implementation does not seem to do that.

Fixes: signed integer overflow: 6317568 * 410 cannot be represented in type 'int'
Fixes: 20492/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_G729_fuzzer-5700189272932352

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 82d4c7b95e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/g729postfilter.c

@@ -600,6 +600,7 @@ int16_t ff_g729_adaptive_gain_control(int gain_before, int gain_after, int16_t *
             gain = ((gain_before - gain_after) << 14) / gain_after + 0x4000;
             gain = bidir_sal(gain, exp_after - exp_before);
         }
+        gain = av_clip_int16(gain);
         gain = (gain * G729_AGC_FAC1 + 0x4000) >> 15; // gain * (1-0.9875)
     } else
         gain = 0;