s3:utils: Fix buffer underflow in parse_timeout for empty strings

s3:utils: Fix buffer underflow in parse_timeout for empty strings

The parse_timeout function is called with a command-line argument. The user
may pass an empty string, which will result in undefined behavior until
the result is checked.

A check for the string length has been added immediately after the strlen()
call. If an empty string is passed, the function returns 0, preventing
incorrect access to the array.

Pair-Programmed-With: Dmitry Mikhalchenko <tascad@altlinux.org>
Signed-off-by: Shumikhina Ksenia <shumikhinaka@sgu.ru>
Reviewed-by: Anoop C S <anoopcs@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Autobuild-User(master): Anoop C S <anoopcs@samba.org>
Autobuild-Date(master): Fri Jun 19 13:03:44 UTC 2026 on atb-devel-224