Do not invoke the xRollbackTo or xRelease methods of a virtual table without

having first invoked an appropriate xSavepoint method.  Add assert() statements
to FTS3/4 to verify that this is happening in all cases.

FossilOrigin-Name: 651ef24249d8c22c4f13e4c0bb98a60099cfd23a

src/sqliteInt.h

@@ -1235,6 +1235,7 @@ struct VTable {
   sqlite3_vtab *pVtab;      /* Pointer to vtab instance */
   int nRef;                 /* Number of pointers to this structure */
   u8 bConstraint;           /* True if constraints are supported */
+  u8 bInSavepoint;          /* True if within a SAVEPOINT */
   VTable *pNext;            /* Next in linked list (see above) */
 };