avcodec/aacdec_template (fixed point): Check gain in decode_cce() to avoid undefined shifts later

Fixes: runtime error: shift exponent 47 is too large for 32-bit type 'int'
Fixes: 2581/clusterfuzz-testcase-minimized-4681474395602944

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