[Fix]Input parsing for modexp precompile

[noel2004]

Mode exp precompile read first 96-bytes of inputs as the expected lengths for the following base, exp and modulus value. In current code, if the input bytes is less than 96, code would wrongly parse the U256 length number with array less than 32 bytes and obtain error value.

For example, if the input has only 1 bytes as '0x01', the code would parse base length as 1, however, the correct value should be 0x01000000000000000000000000000000 (256 bit integer in big-endian).

It cause the failure of test case modexpRandomInput_d1_g0_v0 and has been fixed in this PR.

[noel2004]

modexpRandomInput_d1_g0_v0 has passed:

[2024-01-30T08:22:12Z INFO testool::statetest::executor] modexpRandomInput_d1_g0_v0: mock-prove END
[2024-01-30T08:22:17Z DEBUG testool::statetest::executor] balance_overflow = false
[2024-01-30T08:22:17Z DEBUG testool::statetest::executor] has_l2_different_evm_behaviour_trace = true
[2024-01-30T08:22:17Z WARN testool::statetest::executor] skip post check
[2024-01-30T08:22:17Z INFO testool::statetest::executor] modexpRandomInput_d1_g0_v0: run-test END
[2024-01-30T08:22:17Z INFO testool] result=Ok(())