ccanlint: fix segfault caused by tests not depending on the "info" test.

These tests:

	"depends-exist"      (compulsory_tests/check_depends_exist.c)
	"info-documentation" (tests/has_info_documentation.c)

used m->info_file without checking if it was NULL,
leading to a segfault when no _info file was present.

Some other tests also used m->info_file without depending on "info",
but are taken care of indirectly by this patch.