Fix a bug in the NOT NULL/IS NULL optimization of check-in [cb94350185f555c3]

that can cause invalid data to be used for a column if that column has a
CHECK constraint that includes the NOT NULL or IS NULL operator.
Problem discovered by the 
[https://issues.chromium.org/issues/415397143|Chromium fuzzer].  Never
seen in the wild, as far as anybody knows.

FossilOrigin-Name: 2adaee9aa90f280a406007695fbc4a314806584c93d6b62b46c031492b31ec27

src/vdbe.c

@@ -3711,6 +3711,7 @@ case OP_MakeRecord: {
       zHdr += sqlite3PutVarint(zHdr, serial_type);
       if( pRec->n ){
         assert( pRec->z!=0 );
+        assert( pRec->z!=(const char*)sqlite3CtypeMap );
         memcpy(zPayload, pRec->z, pRec->n);
         zPayload += pRec->n;
       }