security: add zero-address validation and fix pragma consistency

[canychan]

Security Improvements

Changes

1. Zero-address validation in constructor (src/flash.sol)

   • Added require(daiJoin_ != address(0)) to prevent deployment with invalid address
   • Without this check, deploying with address(0) renders the contract permanently unusable

2. Fixed floating pragma (src/base/FlashLoanReceiverBase.sol)

   • Changed to pragma solidity 0.6.12 to match main contract

Impact

• Severity: Low (defensive coding)
• No functional or breaking changes

[DaiFoundation-DevOps]

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