Fix some issues detected by fuzzing #24
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fintelia
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I still want to do more cleanup before merging, but this has fixes for three issues detected by fuzzing. Two were places where the decoder didn't properly reject invalid bitstreams. The final issue is that the color indexing transform was implemented wrong in cases where 16 or fewer colors were in use, causing us to decode incorrectly and possibly index out of bounds. I believe that could result in valid images being decoded garbled.
With this patch applied, fuzzing no longer immediately crashes for me when running with the seed corpus from https://lcamtuf.coredump.cx/afl/demo. However, I haven't had a chance to fuzz for an extended length of time. And cargo fuzz does occasionally print messages about specific inputs taking nearly a minute to run
Edit: I'll handle the color indexing transform issue is a separate PR, hopefully with a regression test. For now, this PR just includes three smaller bitstream issues.