util/encoding: fix BenchmarkPeekLengthDuration
util/encoding: fix BenchmarkPeekLengthDuration

The random durations being created had overflow causing encoding to
fail, but we were ignoring the error. Subsequent calls to PeekLength
were benchmarking returning an error.

name                  old time/op  new time/op  delta
PeekLengthDuration-8  2.26µs ± 2%  0.04µs ± 2%  -98.42%  (p=0.008 n=5+5)

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