Avoid overflow or underflow on fc::safe initialization

[abitmore]

Bug Description
An overflow or underflow can occur when initializing a fc::safe<T> variable.

E.G.

uint64_t a = -1;
fc::safe<int64_t> b = a;

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)

Steps To Reproduce
Steps to reproduce the behavior (example outlined below):

1. Execute API call '...'
2. Using JSON payload '...'
3. Received response '...'
4. See error in screenshot

Expected Behavior
A clear and concise description of what you expected to happen.

Screenshots (optional)
If applicable, add screenshots to help explain process flow and behavior.

Host Environment
Please provide details about the host environment. Much of this information can be found running: witness_node --version.

• Host OS: [e.g. Ubuntu 18.04 LTS]
• Host Physical RAM [e.g. 4GB]
• BitShares Version: [e.g. 2.0.180425]
• OpenSSL Version: [e.g. 1.1.0g]
• Boost Version: [e.g. 1.65.1]

Additional Context (optional)
Add any other context about the problem here.

CORE TEAM TASK LIST

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

[jmjatlanta]

Example: asset_issue_operation::calculate_fee (around line 79). Overflow could happen before implied conversion of return.

Also around line 84. core_fee_required is uint64_t. Overflow could happen on line 96.

[attritionorg]

Is there an attacker controlled path to exploit this?

[abitmore]

Fixed by bitshares/bitshares-fc#248, and bumped FC in #2739.