avcodec/aacdec_fixed: Fix undefined integer overflow in apply_independent_coupling_fixed()

Fixes: signed integer overflow: 1195517 * 2048 cannot be represented in type 'int'
Fixes: 8636/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_FIXED_fuzzer-4695836326887424

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


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