videoio: gst: Fix gst assertion on null msg

videoio: gst: Fix gst assertion on null msg

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>