opus_decode undefined behavior decoding FEC from invalid packet

Attempting to decode FEC from an invalid packet can cause the opus decoder to use uninitialized data from the stack, and potentially dereference out-of-bounds memory addresses or crash depending on the contents of the uninitialized data.

Decoding the attached opus_demo bitstream under valgrind demonstrates the issue.

opus_demo -d 48000 1 -inbandfec opus-fec-uninit.opusdemo out.pcm