Avoid negative bit shift operatoin in huffdec.c (CVE-2024-56431).

A crash was discovered using input fuzzying, in th_decode_ceaderin()
where the len value in the oc_fuff_tree_unpack() can end up as -1.
Added a check to ensure this do not happen.

Based on feedback from Timothy B. Terriberry.

The issue was discovered using gcc sanitazion, which reported the following:

huffdec.c:228:27: runtime error: shift exponent -1 is negative
    #0 0x5d471012bfd0 in oc_huff_tree_unpack /home/uos/libtheora-18570/theora/lib/huffdec.c:228
    #1 0x5d471012c134 in oc_huff_trees_unpack /home/uos/libtheora-18570/theora/lib/huffdec.c:392
    #2 0x5d471010a98c in oc_setup_unpack /home/uos/libtheora-18570/theora/lib/decinfo.c:169
    #3 0x5d471010a98c in oc_dec_headerin /home/uos/libtheora-18570/theora/lib/decinfo.c:238
    #4 0x5d471010a98c in th_decode_headerin /home/uos/libtheora-18570/theora/lib/decinfo.c:266
    #5 0x5d47100fd638 in TheoraDecoder::initialize() /home/uos/libtheora-18570/libtheora-18570/fuzzer.cpp:66
    #6 0x5d47100ffa76 in TheoraDecoder::Run() /home/uos/libtheora-18570/libtheora-18570/fuzzer.cpp:180
    #7 0x5d47100ffe48 in main /home/uos/libtheora-18570/libtheora-18570/fuzzer.cpp:240
    #8 0x7cc9a5e29d8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #9 0x7cc9a5e29e3f in __libc_start_main_impl ../csu/libc-start.c:392
    #10 0x5d47100f9964 in _start (/home/uos/libtheora-18570/libtheora-18570/poc1+0x83964)

Fixes github pull request #19.