Use unsigned values for the final DES left shifts

[dand-oss]

Convert L1 and R1 to ULONG with static_cast before the final left shifts in des_cipher. This preserves the output bits while avoiding shifts of negative signed values in pre-C++20 builds.

The negative-shift UBSan diagnostic is reproducible in C++17. Master now uses C++20, so this PR does not claim the same operation is undefined under master's current language mode; the change also provides a small backportable fix for the Firebird 5 line.

Validation on Linux x86-64:

• The patched production enc.cpp passes GCC 16.2 C++20 syntax compilation with upstream's generated configuration headers.
• A single-threaded harness using the enc.cpp algorithm, with only its Firebird mutex wrapper removed and its integral typedefs supplied, reproduces left shift of negative value -1608722304 on the unpatched code under Clang 21 / C++17 / UBSan.
• Patched C++17 + UBSan and patched C++20 both match system libcrypt for 24,576 password/salt combinations (six passwords, all 4,096 two-character salts). Unpatched C++20 also matches the same reference.
• git diff --check passes.

A complete patched server build has not been completed locally; the algorithm harness does not test Firebird's locking or initialization lifecycle.