Handle missing array descriptor in ada_type_of_array

The test case gdb.ada/mi_var_access.exp was failing with gnat-llvm.
Debugging this, I found that the problem was that with gnat-llvm, the
array descriptor would have a NULL pointer for the bounds when the
array was invalidated.  That is, examining the object in C mode:

    (gdb) p a_string_access
    $1 = {
      P_ARRAY = 0x0,
      P_BOUNDS = 0x0
    }

whereas when using GNAT we see:

    (gdb) print a_string_access
    $1 = {
      P_ARRAY = 0x0,
      P_BOUNDS = 0x402750
    }

This was causing ada_type_of_array to return nullptr; with that
bubbling up to varobj and then MI as a "wrong" type in the MI output.

It seems to me that a null P_BOUNDS is reasonable; and that this case
can be handled in ada_type_of_array by examining the type of P_BOUNDS
without needing the bounds themselves.

The bound values are both set to 0 in this case, because
experimentally this is what is done at runtime in the GNAT-generated
code.  Perhaps an explicitly empty array (1/0) would be better; I am
not certain.