cogl-gst: Fix get_caps implementation when no context is set
cogl-gst: Fix get_caps implementation when no context is set

If no context is set on the CoglGstVideoSink then it would previously
call gst_caps_ref with a NULL pointer. This patch makes it just return
NULL instead. I think that is a valid thing to do because that is what
gst_base_sink_default_get_caps does. If we don't do this then it's not
possible to use CoglGstVideoSink with GstParse because that tries to
link the pipeline after parsing the string. That was previously
causing a critical error because the freshly parsed sink doesn't have
a CoglContext.

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>
Reviewed-by: Lionel Landwerlin <c05e47e57584a5194aa13fa9b87ac49f6ecd5024@gmail.com>

djdeath/cogl,djdeath/cogl,djdeath/cogl