Replicates http://www.sqlite.org/cvstrac/chngview?cn=4151 which

Replicates http://www.sqlite.org/cvstrac/chngview?cn=4151 which
modified fts2:

Modify handling of SQLITE_SCHEMA in fts2 code. An SQLITE_SCHEMA error
may cause SQLite to reload the internal schema, deleting and
recreating v-table objects. So the sqlite3_vtab structure can be
deleted out from under a v-table implementation. (CVS 4183)

FossilOrigin-Name: f9020cffda02923ef45979bb447ec2e232086ad5