gstplayer: Plug a memory leak

gstplayer: Plug a memory leak

This was showing up as a memory leak in GTK's
gstreamer media backend:

40 bytes in 1 blocks are definitely lost in loss record 18,487 of 40,868
   at 0x484586F: malloc (vg_replace_malloc.c:381)
   by 0x50D5278: g_malloc (gmem.c:125)
   by 0x50EDBA5: g_slice_alloc (gslice.c:1072)
   by 0x50EFBCC: g_slice_alloc0 (gslice.c:1098)
   by 0x51F2F45: g_type_create_instance (gtype.c:1911)
   by 0x51DAE37: g_object_new_internal (gobject.c:2011)
   by 0x51DC080: g_object_new_with_properties (gobject.c:2181)
   by 0x51DCB20: g_object_new (gobject.c:1821)
   by 0x9855F86: UnknownInlinedFun (gstplayer-wrapped-video-renderer.c:109)
   by 0x9855F86: gst_player_new (gstplayer.c:579)

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1374

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2875>