Fix a bug in DROP TABLE that could cause SQLITE_MASTER table corruption.

Fix a bug in DROP TABLE that could cause SQLITE_MASTER table corruption.
The root problem was that the sequence of BTree operations (Delete, Next)
would not always leave the cursor pointing at the first entry after the
entry that was deleted.  A consequence of this error was that a DROP TABLE
on a table with indices would not always remove every index associated
with that table from the SQLITE_MASTER table.  Subsequent attempts to
open the database will fail when the index for the missing table was
parsed.  Changes have also been made to ignore extra indices in the
SQLITE_MASTER table so that a database previously corrupted by this bug
is once again readable. (CVS 316)

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