nlesource: Protect seeks from tear down

nlesource: Protect seeks from tear down

Otherwise there is a race where we trigger the seek at the exact
same time the composition is being teared down potentially leading
to basesrc restarting its srcpad task which ends up being leaked.

Fixes ges.playback.scrub_backward_seeking.test_title.audio_video.vorbis_theora_ogg
and probably all its friends timeouting with the following stack trace:

     (gdb) t a a bt

     Thread 4 (Thread 0x7f5962acd700 (LWP 19997)):
     #0  0x00007f5976713efd in syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
     #1  0x00007f5976a9d3f3 in g_cond_wait (cond=cond@entry=0x7f5938125410, mutex=mutex@entry=0x7f59381253c8) at gthread-posix.c:1402
     #2  0x00007f5976c9e26b in gst_task_func (task=0x7f59381253b0 [GstTask]) at ../subprojects/gstreamer/gst/gsttask.c:313
     #3  0x00007f5976a7ecb3 in g_thread_pool_thread_proxy (data=<optimized out>) at gthreadpool.c:307
     #4  0x00007f5976a7e2aa in g_thread_proxy (data=0x7f5954071d40) at gthread.c:784
     #5  0x00007f59767ea58e in start_thread (arg=<optimized out>) at pthread_create.c:486
     #6  0x00007f59767196a3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

     Thread 3 (Thread 0x7f5963fff700 (LWP 19995)):
     #0  0x00007f597670e421 in __GI___poll (fds=0xe32da0, nfds=2, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
     #1  0x00007f5976a553a6 in g_main_context_poll (priority=<optimized out>, n_fds=2, fds=0xe32da0, timeout=<optimized out>, context=0xe31ff0) at gmain.c:4221
     #2  0x00007f5976a553a6 in g_main_context_iterate (context=0xe31ff0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3915
     #3  0x00007f5976a55762 in g_main_loop_run (loop=0xe32130) at gmain.c:4116
     #4  0x00007f59768db10a in gdbus_shared_thread_func (user_data=0xe31fc0) at gdbusprivate.c:275
     #5  0x00007f5976a7e2aa in g_thread_proxy (data=0xe1b8a0) at gthread.c:784
     #6  0x00007f59767ea58e in start_thread (arg=<optimized out>) at pthread_create.c:486
     #7  0x00007f59767196a3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

     Thread 2 (Thread 0x7f5968dcc700 (LWP 19994)):
     #0  0x00007f597670e421 in __GI___poll (fds=0xe1bcc0, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
     #1  0x00007f5976a553a6 in g_main_context_poll (priority=<optimized out>, n_fds=1, fds=0xe1bcc0, timeout=<optimized out>, context=0xe1b350) at gmain.c:4221
     #2  0x00007f5976a553a6 in g_main_context_iterate (context=context@entry=0xe1b350, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3915
     #3  0x00007f5976a554d0 in g_main_context_iteration (context=0xe1b350, may_block=may_block@entry=1) at gmain.c:3981
     #4  0x00007f5976a55521 in glib_worker_main (data=<optimized out>) at gmain.c:5861
     #5  0x00007f5976a7e2aa in g_thread_proxy (data=0xe1b800) at gthread.c:784
     #6  0x00007f59767ea58e in start_thread (arg=<optimized out>) at pthread_create.c:486
     #7  0x00007f59767196a3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

     Thread 1 (Thread 0x7f5975df4fc0 (LWP 19993)):
     #0  0x00007f5976713efd in syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
     #1  0x00007f5976a9d3f3 in g_cond_wait (cond=cond@entry=0xe34020, mutex=0xe39b80) at gthread-posix.c:1402
     #2  0x00007f5976a7f41c in g_thread_pool_free (pool=0xe34000, immediate=0, wait_=<optimized out>) at gthreadpool.c:776
     #3  0x00007f5976c9f1ca in default_cleanup (pool=0xe256b0 [GstTaskPool]) at ../subprojects/gstreamer/gst/gsttaskpool.c:89
     #4  0x00007f5976c9e32d in init_klass_pool (klass=<optimized out>) at ../subprojects/gstreamer/gst/gsttask.c:161
     #5  0x00007f5976c9e502 in gst_task_cleanup_all () at ../subprojects/gstreamer/gst/gsttask.c:381
     #6  0x00007f5976c214f4 in gst_deinit () at ../subprojects/gstreamer/gst/gst.c:1095
     #7  0x000000000040394f in main (argc=6, argv=<optimized out>) at ../subprojects/gst-editing-services/tools/ges-launch.c:94