avcodec/flacdec: Fix overflow in multiplication in decode_subframe_fixed()

Fixes: signed integer overflow: 2 * 1629495328 cannot be represented in type 'int'
Fixes: 4716/clusterfuzz-testcase-minimized-5835915940331520

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