Fixes logic error in Decimal::precision #177

Job	Run time
Setup Build Matrix (false, ion-rust, windows, macos, ubuntu, al2-x86, al2-arm, windows, macos, ub...	1s
Build and Test (windows, default)	2m 9s
Build and Test (windows, all)	4m 40s
Build and Test (windows, experimental-ion-hash)	2m 50s
Build and Test (windows, experimental)	3m 4s
Build and Test (macos, default)	52s
Build and Test (macos, all)	1m 16s
Build and Test (macos, experimental-ion-hash)	1m 16s
Build and Test (macos, experimental)	55s
Build and Test (ubuntu, default)	1m 14s
Build and Test (ubuntu, all)	1m 50s
Build and Test (ubuntu, experimental-ion-hash)	57s
Build and Test (ubuntu, experimental)	1m 4s
Build and Test Confirmation	1s
	22m 9s

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