gzip: fix bug in unpack EOB check

Problem reported by Vidar Holen (Bug#28861).
* NEWS: Mention fix.
* tests/unpack-valid: New test.
* tests/Makefile.am (TESTS): Add it.
* unpack.c (build_tree): Report an error if Huffman tree has
too few leaves.
* unpack.c (unpack): Fix check for EOB.
Remove now-unnecessary check for code out of range.