net/samba416: Patch to prevent abnormal smbd abort

Update a call to memcpy() because readdir() only guarantees memory up to
result+result->d_reclen is readable. Under certain conditions,
result+sizeof(struct dirent) landed in unmapped memory.

Most of the legwork to pinpoint the problem, as well as a solution
similar to the one applied here, was submitted by uratan@miomio.jp.
Martin Simmons <martin@lispworks.com> contributed to understanding the
problem and wrote a useful test case.

PR: 275597
Approved by: maintainer timeout
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43171