simplify ada-valprint.c:ada_val_print_array

simplify ada-valprint.c:ada_val_print_array

Two things:
  - Move the declaration of a couple of variables inside the block
    where they are actually used;
  - Remove some code that checks against NULL/zero, because the
    condition should always be false. Add some gdb_asserts to
    make sure we never fail those assumptions.

gdb/ChangeLog:

        * ada-valprint.c (ada_val_print_array): Move the declaration of
        "byte_order" and "elttype" inside the block where these variables
        are actually used.  Remove some special handling for the case
        where "elttype" and "eltlen" are null.  Replace by a comment
        and a couple of assertion checks.