tests: fix race in streamsynchronizer test

Wait for thread to exit before starting to free the
to_push list, otherwise thread might check the final
to_push->next node only after we've freed it already.
rtpbuffer: Fix gst_rtp_buffer_ext_timestamp() with clang 5 on iOS/ARM

The bitwise NOT operator is not defined on signed integers.
Thanks to Wim Taymans for finding the cause.

https://bugzilla.gnome.org/show_bug.cgi?id=711819
typefind: Fix identification of some MPEG files

Make sure we begin by peeking at MPEG2_MAX_PROBE_LENGTH
bytes.

Fixes:

https://bugzilla.gnome.org/show_bug.cgi?id=678011
tests: fix appsrc test with latest GLib version

With the latest GLib, g_source_remove() complains about not finding
the timeout source with the given ID here, since it was already
destroyed by returning FALSE from the timeout callback. Also return
FALSE from the bus watches when we don't want to be called any more.
tests: fix input-selector-test

Update for pad template name changes.
subparse: g_memmove() is deprecated

Just use plain memmove(), g_memmove() is deprecated in
recent GLib versions.

https://bugzilla.gnome.org/show_bug.cgi?id=712811
subparse: fix null pointer access in sami parser

https://bugzilla.gnome.org/show_bug.cgi?id=712805
playsink: Fix visualizations if no visualization plugin was set

https://bugzilla.gnome.org/show_bug.cgi?id=712280
videodecoder: avoid descending output timestamps

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=712796