gio: cleanup gdbusmessage.c file

* removed passing GError to ensure_input_padding() function
  - it was necessary before commit 3e5214c15c2a19b64d74e714
  when we used GData*Streams and GMemoryInputStream with
  g_seekable_seek() - now it's useless,

* removed checking return value of ensure_input_padding()
  function - in previous implementation (like above)
  g_seekable_seek() could return FALSE - now it's always TRUE,

* removed passing GError to g_memory_buffer_read_*() functions
  and checking returned value - it also has been inherited after
  old implementation with g_data_input_stream_read_*() functions
  - now it's also useless

* cleaned up code formatting,

https://bugzilla.gnome.org/show_bug.cgi?id=729875