sql: fix fkey exception for self-referenced table

UPDATE operation doesn't fail when fkey self-reference condition
unsatisfied and table has other records.
To do not raise error where it is not necessary Vdbe inspects
parent table with OP_Found. This branch is not valid
for self-referenced table since its looking for a tuple affected
by UPDATE operation and since the foreign key has already
detected a conflict it must be raised.

Example:
CREATE TABLE t6(a INTEGER PRIMARY KEY, b TEXT, c INT, d TEXT, UNIQUE(a, b),
                FOREIGN KEY(c, d) REFERENCES t6(a, b));
INSERT INTO t6 VALUES(1, 'a', 1, 'a');
INSERT INTO t6 VALUES(100, 'one', 100, 'one');
UPDATE t6 SET c = 1, d = 'a' WHERE a = 100;
-- fk conflict must be raised here

Needed for #3850
Closes #3918