avcodec/dirac_dwt_template: Fix undefined behavior in interleave()

Fixes: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
Fixes: 8697/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5197148130902016

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


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