Enhancements to covering-index prediction. In particular, avoid the

Enhancements to covering-index prediction.  In particular, avoid the
false-positive prediction described by
[forum:/forumpost/e60e4c295d22f8ce|forum post e60e4c295d22f8ce].
Add early detection of over-prediction of covering-indexes so that
sqlite3_prepare() will return an error rather than just generate bad
bytecode.

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