diagnose unexpected EOF and zero lengths in packed data

diagnose unexpected EOF and zero lengths in packed data

Problem reported by Aki Helin.
* NEWS: Mention Aki's reports.
* tests/unpack-invalid: New file,
with test data suggested by Aki.
* tests/Makefile.am (TESTS): Add it.
* unpack.c (read_byte): New function.
(look_bits, read_tree): Use it.
(read_tree): Check against zero bit length Huffman code.