Fix undefined behavior exposed in scalar_multiplication.cpp.

[codygunton]

Exposed here: https://github.com/AztecProtocol/aztec-packages/pull/6261

[ RUN      ] GoblinRecursionTests.Vanilla
/mnt/user-data/cody/aztec-packages/barretenberg/cpp/src/barretenberg/ecc/scalar_multiplication/scalar_multiplication.cpp:345:42: runtime error: addition of unsigned offset to 0x7a2eccd8e040 overflowed to 0x7a2eccd8e020
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /mnt/user-data/cody/aztec-packages/barretenberg/cpp/src/barretenberg/ecc/scalar_multiplication/scalar_multiplication.cpp:345:42 in 
/mnt/user-data/cody/aztec-packages/barretenberg/cpp/src/barretenberg/ecc/scalar_multiplication/scalar_multiplication.cpp:345:42: runtime error: addition of unsigned offset to 0x6206189a6800 overflowed to 0x6206189a67e0
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /mnt/user-data/cody/aztec-packages/barretenberg/cpp/src/barretenberg/ecc/scalar_multiplication/scalar_multiplication.cpp:345:42 in 
[       OK ] GoblinRecursionTests.Vanilla (32686 ms)