Fixes logic error in Decimal::precision (#797) #178

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)	4m 37s
Build and Test (windows, all)	4m 56s
Build and Test (windows, experimental-ion-hash)	2m 55s
Build and Test (windows, experimental)	2m 56s
Build and Test (macos, default)	1m 2s
Build and Test (macos, all)	56s
Build and Test (macos, experimental-ion-hash)	51s
Build and Test (macos, experimental)	56s
Build and Test (ubuntu, default)	53s
Build and Test (ubuntu, all)	1m 48s
Build and Test (ubuntu, experimental-ion-hash)	1m 0s
Build and Test (ubuntu, experimental)	1m 1s
Build and Test Confirmation	1s
	23m 53s

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