Add workaround for invalid validator instance after validation error

[k1LoW]

ref: k1LoW/runn#884

[github-actions]

Code Metrics Report

	main (79ff03e)	#47 (a277ee7)	+/-
Coverage	60.3%	60.5%	+0.2%
Code to Test Ratio	1:0.8	1:0.8	-0.0

Details

  |                    | main (79ff03e) | #47 (a277ee7) |  +/-  |
  |--------------------|----------------|---------------|-------|
+ | Coverage           |          60.3% |         60.5% | +0.2% |
  |   Files            |              4 |             4 |     0 |
  |   Lines            |            569 |           582 |   +13 |
+ |   Covered          |            343 |           352 |    +9 |
- | Code to Test Ratio |          1:0.8 |         1:0.8 |  -0.0 |
  |   Code             |           1075 |          1093 |   +18 |
  |   Test             |            825 |           825 |     0 |

Code coverage of files in pull request scope (54.2% → 56.5%)

Files	Coverage	+/-
openapi3.go	56.5%	+2.3%

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Reported by octocov