Unnecessary array boundaries check when loading an array element twice

[code423n4]

Handle

robee

Vulnerability details

There are places in the code (especially in for-each loops) that loads the same array element more
than once. In such cases, only one array boundaries check should take place, and the rest are unnecessary.
Therefore, this array element should be cached in a local variable and then be loaded
again using this local variable, skipping the redundent second array boundaries check:

    PieFactoryContract.sol, variable name: _tokens times: 2 at: bakePie
    SingleNativeTokenExit.sol, variable name: tokens times: 2 at: exitEth
    SingleTokenJoin.sol, variable name: tokens times: 2 at: _joinTokenSingle
    SingleTokenJoin.sol, variable name: amounts times: 2 at: _joinTokenSingle
    SingleTokenJoinV2.sol, variable name: inputs times: 4 at: _joinTokenSingle