Fee amount validation

[abitmore]

Bug Description
Most non-virtual operations have a check FC_ASSERT(fee.amount >= 0) in validate() function. But some are different.

• blind_transfer_operation doesn't check fee.amount;
• proposal_create_operation doesn't check fee.amount;
• custom_operation checks fee.amount > 0, which is practically fine but not perfect;
• balance_claim_operation checks fee.amount == asset(), which is fine.

Anyway, there is a check FC_ASSERT( fee.amount >= 0 ) in generic_evaluator::prepare_fee(), so the chain won't allow any user to pay a negative fee, which means we're safe. However, since the check is done in evaluators, there exists a minor issue that a user can wrap an operation with a negative fee in a proposal_create_operation which could be accepted by the chain, just a bit annoying.

Impacts
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.

• API (the application programming interface)
• Build (the build process or something prior to compiled code)
• CLI (the command line wallet)
• Deployment (the deployment process after building such as Docker, Travis, etc.)
• DEX (the Decentralized EXchange, market engine, etc.)
• P2P (the peer-to-peer network for transaction/block propagation)
• Performance (system or user efficiency, etc.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• Security (the security of system or user data, etc.)
• UX (the User Experience)
• Other (please add below)

CORE TEAM TASK LIST

• Evaluate / Prioritize Bug Report
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation