Problem reported by Gisle Vanem <https://bugs.gnu.org/64316>.
* src/io.c (find_and_hash_each_line):
Rely on xpalloc to check for integer overflow instead
of trying to do it ourselves incorrectly, with old code
that predated the use of xpalloc.
* src/system.h: Verify that LIN_MAX == IDX_MAX,
since the code now relies on this.
* tests/Makefile.am (TESTS): Add bug-64316.
* tests/bug-64316: New file
