avcodec/shorten: Fix integer overflow in residual/LPC combination

Fixes: signed integer overflow: -540538872 + -2012739576 cannot be represented in type 'int'
Fixes: 9255/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5758630052757504

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 db7e9082e1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/shorten.c

@@ -315,7 +315,7 @@ static int decode_subframe_lpc(ShortenContext *s, int command, int channel,
         for (j = 0; j < pred_order; j++)
             sum += coeffs[j] * (unsigned)s->decoded[channel][i - j - 1];
         s->decoded[channel][i] = get_sr_golomb_shorten(&s->gb, residual_size) +
-                                 (sum >> qshift);
+                                 (unsigned)(sum >> qshift);
     }
 
     /* add offset to current samples */