libiberty: avoid reading past end of buffer in strndup/xstrndup (PR c/78498)

gcc/ChangeLog:
	PR c/78498
	* selftest.c (selftest::assert_strndup_eq): New function.
	(selftest::test_strndup): New function.
	(selftest::test_libiberty): New function.
	(selftest::selftest_c_tests): Call test_libiberty.

gcc/testsuite/ChangeLog:
	PR c/78498
	* gcc.dg/format/pr78494.c: New test case.

libiberty/ChangeLog:
	PR c/78498
	* strndup.c (strlen): Delete decl.
	(strnlen): Add decl.
	(strndup): Call strnlen rather than strlen.
	* xstrndup.c (xstrndup): Likewise.

From-SVN: r243030