Don't close passed in FileStream handles in constructor (dotnet/coreclr#12253)

Don't close passed in FileStream handles in constructor (dotnet/coreclr#12253)

* Don't close passed in FileStream handles in constructor

As the finalizer will run when throwing we need to be
careful to make sure we don't put the passed-in handle
in the member field until we're sure we've succeeeded.

Additionally we need to make sure any wrapping SafeHandle
we create on a passed in IntPtr isn't collected.

* Move platform specific assertion and add comment.

* Comment- fix mistake in CanSeekCore

Commit migrated from https://github.com/dotnet/coreclr/commit/8a8bda14a618b14264cb85c012e097b3e01992e9