New assert() statements to validate the parameters to

sqlite3BtreeCursorHint().  Fix a problem with the construction of those
parameters discovered by
[forum:/forumpost/0b53708c95|forum post 0b53708c95].

FossilOrigin-Name: 4c5a3c5fb351cc1c2ce16c33314ce19c53531f09263f87456283d9d756002f9d

src/sqliteInt.h

@@ -4186,6 +4186,7 @@ struct Walker {
     struct CoveringIndexCheck *pCovIdxCk;     /* Check for covering index */
     SrcItem *pSrcItem;                        /* A single FROM clause item */
     DbFixer *pFix;                            /* See sqlite3FixSelect() */
+    Mem *aMem;                                /* See sqlite3BtreeCursorHint() */
   } u;
 };