Fix MODEXP precompile success validation

[k06a]

MODEXP precompile always return success, but returndatasize() is 0 on failure.

Few more things to consider:

• Include sub(n, 2) underflow check into success: success := and(gt(n, 1), eq(returndatasize(), 0x20))
• Using 0 instead of freemem for call output memory pointer will be few gas cheaper