s3: lib: Paranoia around use of snprintf copying into a fixed-size buffer from a...

s3: lib: Paranoia around use of snprintf copying into a fixed-size buffer from a getenv() pointer.

Post checks for overflow/error.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Mon May 18 23:42:57 UTC 2020 on sn-devel-184

(cherry picked from commit dd1f750293ef4361455a5d5b63fc7a89495715b7)

Autobuild-User(v4-10-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-10-test): Fri May 22 16:07:51 UTC 2020 on sn-devel-144