gst/gstminiobject.c: Some cleanup and checking against invalid function parameters.

gst/gstminiobject.c: Some cleanup and checking against invalid function parameters.

Original commit message from CVS:
* gst/gstminiobject.c: (gst_mini_object_get_type),
(gst_mini_object_class_init), (gst_mini_object_copy_default),
(gst_mini_object_finalize), (gst_mini_object_copy),
(gst_mini_object_is_writable), (gst_mini_object_make_writable),
(gst_mini_object_replace), (param_mini_object_validate),
(gst_param_spec_mini_object_get_type):
Some cleanup and checking against invalid function parameters.