Have the dbstat virtual table take a copy of each page buffer that it traverses inste...

Have the dbstat virtual table take a copy of each page buffer that it traverses instead of just a reference to the page-cache object. This avoids problems if an error causes transaction rollback while a dbstat cursor is open. dbsqlfuzz crash-417224040fee04f0f0e62b70265c518893b08769.

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