Fix uudecode buffer overflow.
mutt_decode_uuencoded() used each line's initial "length character" without any validation. It would happily read past the end of the input line, and with a suitable value even past the length of the input buffer. As I noted in ticket 404, there are several other changes that could be added to make the parser more robust. However, to avoid accidentally introducing another bug or regression, I'm restricting this patch to simply addressing the overflow. Thanks to Tavis Ormandy for reporting the issue, along with a sample message demonstrating the problem.