Add overflow when decoding 12 bit RGB

[sdroege]

thread '<unnamed>' panicked at 'attempt to add with overflow', /home/slomo/.cargo/git/checkouts/ffv1-08f2e3b26709fb34/2afb025/src/jpeg2000rct.rs:74:58
stack backtrace:
   0: rust_begin_unwind
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/std/src/panicking.rs:515:5
   1: core::panicking::panic_fmt
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:50:5
   3: <u16 as ffv1::jpeg2000rct::Rct<u8>>::rct
   4: ffv1::decoder::Decoder::decode_slice_content
             at /home/slomo/.cargo/git/checkouts/ffv1-08f2e3b26709fb34/2afb025/src/decoder.rs:714:17
   5: ffv1::decoder::Decoder::decode_slice
             at /home/slomo/.cargo/git/checkouts/ffv1-08f2e3b26709fb34/2afb025/src/decoder.rs:826:9
   6: ffv1::decoder::Decoder::decode_frame
             at /home/slomo/.cargo/git/checkouts/ffv1-08f2e3b26709fb34/2afb025/src/decoder.rs:245:23
   7: <gstffv1::ffv1dec::imp::Ffv1Dec as gstreamer_video::subclass::video_decoder::VideoDecoderImpl>::handle_frame
             at ./src/ffv1dec/imp.rs:377:29

See attached file: test.mkv.zip