Fix an assert() in the code generator for expressions so that it is valid

even after an OOM.

FossilOrigin-Name: 427e83eb583a4f63650912f078122e7373fe8c7ea11acfdab464d748b4235261

src/expr.c

@@ -4016,7 +4016,7 @@ expr_code_doover:
       ** Expr node to be passed into this function, it will be handled
       ** sanely and not crash.  But keep the assert() to bring the problem
       ** to the attention of the developers. */
-      assert( op==TK_NULL );
+      assert( op==TK_NULL || pParse->db->mallocFailed );
       sqlite3VdbeAddOp2(v, OP_Null, 0, target);
       return target;
     }