VS: Avoid crash with VS 2015 when all SDKs are higher than 10.0.14393.0

Move the filter added by commit v3.13.0-rc1~72^2~2 (VS: Do not select a
Windows SDK too high for current VS version, 2017-08-07, committed
2018-09-17) to before our check that the remaining list is empty.
Otherwise we crash when dereferencing the first entry of an empty
vector.

Also add a comment explaining where 10.0.14393.0 came from.

Fixes: #18633
Status Job ID Name Coverage
  External
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