Replace segfault with actual error message

src/process_pipeline.c

@@ -835,6 +835,11 @@ process_capture_burst(GdkTexture *thumb)
         bool save_dng = g_settings_get_boolean(settings, "save-raw");
         char *postprocessor = g_settings_get_string(settings, "postprocessor");
 
+        if (postprocessor == NULL) {
+                g_printerr("Postprocessor setting is null\n");
+                return;
+        }
+
         char save_dng_s[2] = "0";
         if (save_dng) {
                 save_dng_s[0] = '1';