Fix flashloan fee check

[guoyiteng]

As I reported in #344, this fee check is wrong. You can reproduce it by running the unit test in this PR.

[codecov]

Codecov Report

Merging #345 (583173f) into master (b0c731c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files          10       10           
  Lines         505      505           
  Branches       85       85           
=======================================
  Hits          502      502           
  Misses          3        3           

Impacted Files	Coverage Δ
contracts/SwapFlashLoan.sol	100.00% <100.00%> (ø)

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[alphastorm]

Hi @guoyiteng, thanks for filing this PR!

We recently launched a bug bounty program on Immunefi and even though this was reported prior, we'd like to honor it as a low level report with a $1,000 USDC bounty (classified per the Immunefi Vulnerability Severity Classification System).

Could you please join our Discord and message alphast0rm#3577 to coordinate? Thanks again!

[guoyiteng]

Thank you so much for the bouty! I was thinking that my bug report timing was bad and I missed the bounty. I'm glad that you can honor it. To verify my identity in discord, the secret word is 1337.