[Bug 751528] mpegdemux: Fix a prevent defect which dereference null return value. (Minor clean up)

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=751528

--- Comment #8 from Sangkyu Park <sk1122.park at samsung.com> ---
It is a prevent defect which dereference null return value.
'gst_ps_demux_get_stream' function could return NULL when it is unknown stream.
So I added NULL check.

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