According to the gstreamer docs [1], the GstMessage pointer returned by
gst_bus_pop() is nullable, meaning NULL is a valid return value.
Previously, gst_is_missing_plugin_message would throw an assert when its
message object parameter would fail the GST_IS_MESSAGE macro check,
crashing the entire process (unless running in a try-catch block of course).
Instead of relying on valid messages, check if the message object itself is
valid before passing it to other gstreamer functions.
[1] https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstBus.html#gst-bus-pop
Signed-off-by: Christopher N. Hesse <raymanfx@gmail.com>
Christopher N. Hesse
796b0fec7d