Add assert() statements and eliminate needless variable assignments in order

to get the clang scan-build utility to report zero problems against the
SQLite core.  Clang's static analysis did find one real problem - but it was
in the command-line shell, not in the SQLite core.

FossilOrigin-Name: 60fee9574b0125705787e33c16f116cf188c8323

src/fkey.c

@@ -1124,6 +1124,7 @@ static Trigger *fkActionTrigger(
       fkTriggerDelete(db, pTrigger);
       return 0;
     }
+    assert( pStep!=0 );
 
     switch( action ){
       case OE_Restrict: