The addition is moved up into the context where the variable is unsigned avoiding
the undefined behavior

Fixes: runtime error: signed integer overflow: 2147481972 + 4096 cannot be represented in type 'int'
Fixes: 12444/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ILBC_fuzzer-5755706244857856

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