msgunfmt: Detect more cases of invalid .mo files.

msgunfmt: Detect more cases of invalid .mo files.

* gettext-tools/src/read-mo.c: Include hash-string.h.
(read_mo_file): Verify the array of messages and the hash table.
* gettext-tools/src/write-mo.c (write_table): Fix typo in comment.
* gettext-tools/src/Makefile.am (msgunfmt_SOURCES): Add hash-string.c.
* gettext-tools/tests/overflow-7.mo: Renamed from
gettext-tools/tests/sysdep-without-nul.mo.
* gettext-tools/tests/sysdep-without-nul.mo: New file.
* gettext-tools/tests/msgunfmt-3: Test also overflow-7.mo.
* gettext-tools/tests/Makefile.am (EXTRA_DIST): Add overflow-7.mo.