Merge pull request #20801 from JonasVautherin:fix-gst-error-handling

* Fix gst error handling

* Use the return value instead of the error, which gives no guarantee of being NULL in case of error
* Test err pointer before accessing it
* Remove unreachable code

* videoio(gstreamer): restore check in writer code
Co-authored-by: NAlexander Alekhin <alexander.a.alekhin@gmail.com>

modules/videoio/src/cap_gstreamer.cpp

@@ -761,7 +761,7 @@ bool GStreamerCapture::open(const String &filename_)
             }
             else
             {
-                CV_WARN("Error opening file: " << filename << " (" << err->message << ")");
+                CV_WARN("Error opening file: " << filename << " (" << (err ? err->message : "<unknown reason>") << ")");
                 return false;
             }
         }
@@ -769,9 +769,9 @@ bool GStreamerCapture::open(const String &filename_)
         {
             GSafePtr<GError> err;
             uridecodebin.attach(gst_parse_launch(filename, err.getRef()));
-            if (err)
+            if (!uridecodebin)
             {
-                CV_WARN("Error opening bin: " << err->message);
+                CV_WARN("Error opening bin: " << (err ? err->message : "<unknown reason>"));
                 return false;
             }
             manualpipeline = true;
@@ -1979,7 +1979,7 @@ void handleMessage(GstElement * pipeline)
                 gst_message_parse_error(msg, err.getRef(), debug.getRef());
                 GSafePtr<gchar> name; name.attach(gst_element_get_name(GST_MESSAGE_SRC (msg)));
                 CV_WARN("Embedded video playback halted; module " << name.get() <<
-                        " reported: " << err->message);
+                        " reported: " << (err ? err->message : "<unknown reason>"));
                 CV_LOG_DEBUG(NULL, "GStreamer debug: " << debug.get());
 
                 gst_element_set_state(GST_ELEMENT(pipeline), GST_STATE_NULL);