Fixes: signed integer overflow: 777777776 * 4 cannot be represented in type 'int'
Fixes: 29102/clusterfuzz-testcase-minimized-ffmpeg_dem_RPL_fuzzer-6726188921913344
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>