ReplicatedPG::fill_in_copy_get: fix early return bug

This is not a leak, we are in the else block where cb must
be NULL.  The fix as introduced did not include braces on
the if causing the method to return unconditionally.

Fixes: #7604
Introduced in: 500206d809f0cd85cd99e4f0ec164bbf74f92c28
Signed-off-by: Samuel Just <sam.just@inktank.com>