Replace the `assert` in `ext.hpp` with `FC_ASSERT`

**User Story**
@pmconrad reported in https://github.com/bitshares/bitshares-fc/pull/126#issuecomment-491578337:
> We should replace the `assert` in `ext.hpp` with `FC_ASSERT` though, to catch misspelled arguments.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [x] 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.)
- [x] UX (the User Experience)
- [ ] Other (please add below)

**Additional Context (optional)**
Add any other context about your request here.

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Open ext.hpp and locate the existing assert mentioned in the issue; then inspect the project’s FC_ASSERT usage to understand the expected replacement. Done means the assert is replaced with FC_ASSERT and the API-facing behavior remains consistent with the request.

Written by the indexing model from the issue text.