avcodec/diracdec: Fix integer overflow in INTRA_DC_PRED()

Fixes: runtime error: signed integer overflow: 1168175789 + 1168178473 cannot be represented in type 'int'
Fixes: 3081/clusterfuzz-testcase-minimized-4807564879462400
Fixes: 2844/clusterfuzz-testcase-minimized-5561715838156800

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