Ensure that the window function rewrite does not leave the parse tree

Ensure that the window function rewrite does not leave the parse tree
in an invalid state that might cause problems downstream before the
error is recognized and unwinds the stack.  Also take steps such that
an invalid parse tree does not cause problems even if it goes
unrecognized.
[forum:/forumpost/398e9d5aa9|Forum post 398e9d5aa9].

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