gst/gdp/: Handle error cases when calling functions do downwards state change after...

gst/gdp/: Handle error cases when calling functions do downwards state change after parent's change_state

Original commit message from CVS:
* gst/gdp/gstgdpdepay.c: (gst_gdp_depay_chain),
(gst_gdp_depay_change_state):
* gst/gdp/gstgdpdepay.h:
* gst/gdp/gstgdppay.c: (gst_gdp_pay_reset_streamheader),
(gst_gdp_pay_chain), (gst_gdp_pay_sink_event),
(gst_gdp_pay_change_state):
* gst/gdp/gstgdppay.h:
Handle error cases when calling functions
do downwards state change after parent's change_state
* tests/check/elements/gdpdepay.c: (GST_START_TEST):
* tests/check/elements/gdppay.c: (GST_START_TEST):
clean up more