Take care not to modify the sqlite* pointer to sqlite_exec() if we suspect

Take care not to modify the sqlite* pointer to sqlite_exec() if we suspect
that the pointer is stale - that it has previously been passed to
sqlite_close().  Possible fix for ticket #202.  Prior to this fix, test
misuse-5.3 was causing a change to a buffer that had been previously free()-ed. (CVS 799)

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