Update varchar.rs to avoid UB #699

Job	Run time
run-guard	1s
linting	19s
testing (ubuntu-20.04, v3, linking)	50s
security	9s
embedded (linking)	41s
testing (ubuntu-20.04, v3, dynamic_loading)	1m 1s
embedded (dynamic_loading)	34s
testing (ubuntu-20.04, v3, pure_rust)	4m 28s
testing (ubuntu-20.04, v2, linking)	1m 0s
testing (ubuntu-20.04, v2, dynamic_loading)	1m 16s
testing (ubuntu-20.04, v2, pure_rust)	4m 21s
testing (ubuntu-20.04, v4, linking)	51s
testing (ubuntu-20.04, v4, dynamic_loading)	48s
testing (ubuntu-20.04, v4, pure_rust)	4m 23s
testing (windows-latest, v4, linking)	1m 17s
testing (windows-latest, v4, dynamic_loading)	1m 11s
testing (windows-latest, v4, pure_rust)	1m 30s
testing (macos-latest, v3, linking)	16s
testing (macos-latest, v3, dynamic_loading)	16s
testing (macos-latest, v3, pure_rust)	16s
coverage	0s
	25m 28s

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