[stdlib] fix an indexing inconsistency
[stdlib] fix an indexing inconsistency

This iteration ranged from `buffer.startIndex` to `buffer.count`, rather than to `buffer.endIndex`.
Using `buffer.indices` is a better solution in any case.

JGiola/swift,apple/swift,JGiola/swift,apple/swift,benlangmuir/swift,glessard/swift,glessard/swift,JGiola/swift,apple/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,roambotics/swift,roambotics/swift,apple/swift,glessard/swift,roambotics/swift,glessard/swift,glessard/swift,apple/swift,benlangmuir/swift,roambotics/swift,apple/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,JGiola/swift,glessard/swift,JGiola/swift