Minor improvements for SplitToOem

[lhecker]

When working on #14745 I noticed that SplitToOem was in a bit of a poor state
as well. Instead of simply iterating over its deque argument and writing the
results into a new deque it used pop to advance the head of both queues.
This isn't quite exception safe and rather bloaty. Additionally there's no need
to call WideCharToMultiByte twice on each character if we know that the most
verbose encoding is UTF-8 which can't be any more than 4 chars anyways.

Related to #8000.

PR Checklist

• 2 unit tests cover this ✅

[carlos-zamora]

@msftbot merge this in 10 minutes