bugfix : when modifying a biblio, on a repeated field with many subfields, the test...

bugfix : when modifying a biblio, on a repeated field with many subfields, the test to show empty subfields in X>1 was wrong : the missing subfields of the 1st field were repeated.
Example :
606 has $a$x$z

1st is 606$a (so $x$z empty)
2nd is 606$a$x

The marc editor in the 2nd field showed before this fix :
606 $a a_value
   $x another_value
   $x
   $z
(the 2nd $x being useless