Fix undefined-struct bug

Set ctype to be defined before inserting into ctype array.
This patch is used to make 'ffi.new("struct a[1]")' parsed successfully.

Signed-off-by: Yicheng Qin <qycqycqycqycqyc@gmail.com>
Add new part in ffi_test.kp

The new part is to test array subscription and struct reference.

Signed-off-by: Yicheng Qin <qycqycqycqycqyc@gmail.com>