gst/playback/: More leak and compile fixes.

gst/playback/: More leak and compile fixes.

Original commit message from CVS:
* gst/playback/gstdecodebin.c: (remove_element_chain):
* gst/playback/gstplaybin.c: (add_sink):
* gst/playback/gststreaminfo.c: (gst_stream_info_dispose),
(gst_stream_info_set_mute):
* gst/playback/gststreamselector.c:
(gst_stream_selector_get_linked_pad),
(gst_stream_selector_getcaps), (gst_stream_selector_chain):
More leak and compile fixes.