avcodec/takdec: Fix integer overflow in decode_lpc()

Fixes: runtime error: signed integer overflow: 16748560 + 2143729712 cannot be represented in type 'int'
Fixes: 3202/clusterfuzz-testcase-minimized-4988291642294272

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