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>

modules/videoio/src/cap_gstreamer.cpp

@@ -1811,6 +1811,10 @@ void handleMessage(GstElement * pipeline)
 
     while(gst_bus_have_pending(bus)) {
         msg = gst_bus_pop(bus);
+        if (!msg || !GST_IS_MESSAGE(msg))
+        {
+            continue;
+        }
 
         //printf("\t\tGot %s message\n", GST_MESSAGE_TYPE_NAME(msg));