Add the OP_FinishSeek opcode which completes an OP_DeferredSeek if the seek

has not already completed.  Also add the sqlite3WhereUsesDeferredSeek()
interface to the query planner.  The UPDATE implementation adds an
OP_FinishSeek before running the final OP_Insert if one is needed.
Ticket [ec8abb025e78f40c] and also an assertion fault reported by Yongheng.

FossilOrigin-Name: 21ef6e99331210b80fa7c71b4f02e8f768a748d01aef884368af2f6b51a067e0

src/btree.c

@@ -8658,7 +8658,6 @@ int sqlite3BtreeInsert(
     if( flags & BTREE_SAVEPOSITION ){
       assert( pCur->curFlags & BTCF_ValidNKey );
       assert( pX->nKey==pCur->info.nKey );
-      assert( pCur->info.nSize!=0 );
       assert( loc==0 );
     }
 #endif