<disclaimer>clueless about GStreamer internals</disclaimer>

valgrind tells me[1] that gstplaybasebin leaks the mutex... from a quick look at
the code I see that gstplaybasebin implements dispose but not finalize: I think
it should implement finalize instead of dispose (dispose may run more than once)
and use finalize to free both the strings currently freed in dispose and the mutex.

Patch follows: UNTESTED


[1]:
==27776== 24 bytes in 1 blocks are definitely lost in loss record 8213 of 14968
==27776==    at 0x3414A5C1: malloc (vg_replace_malloc.c:131)
==27776==    by 0x35192136: IA__g_malloc (gmem.c:137)
==27776==    by 0x35132512: g_mutex_new_posix_impl (gthread-posix.c:151)
==27776==    by 0x35BC037F: gst_play_base_bin_init (gstplaybasebin.c:251)
==27776==    by 0x350EDECA: IA__g_type_create_instance (gtype.c:1588)
==27776==    by 0x350D97CE: g_object_constructor (gobject.c:1052)
==27776==    by 0x3446A2E7: gst_object_constructor (gstobject.c:197)