asan: print_vms_time signed integer overflow

I really don't think anyone cares about underflow of vms time values,
but the potential segfault on a gmtime failure is worth fixing.

	* readelf.c (INT64_MIN): Define if not already defined.
	(print_vms_time): Catch 64-bit overflow when converting from
	vms time to posix time.  Don't segfault if gmtime returns NULL.