qmlglsink: check qt_context_ first in GstQSGTexture::bind()
qmlglsink: check qt_context_ first in GstQSGTexture::bind()

When start qmlglsink app, it will set NULL buffer to GstQSGTexture
in which case that qt_context_ will be a random value and cause
gst_gl_context_activate() fail.

https://bugzilla.gnome.org/show_bug.cgi?id=770925

pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good