ScreenSaverRunner: Fix segmentation fault loading bad screen saver.

When Settings.ModuleName() returned NULL here we got a segmentation fault
trying to access the pointer.

I ran into this trying to load up a non-screensaver executable in ScreenSaver,
it crashed instead of doing the right thing and refusing to load.

Also, since we go to the trouble of saving this to a variable, we should actually use it.

src/kits/screensaver/ScreenSaverRunner.cpp

@@ -96,7 +96,7 @@ ScreenSaverRunner::_LoadAddOn()
 	_CleanUp();
 
 	const char* moduleName = fSettings.ModuleName();
-	if (*moduleName == '\0') {
+	if (moduleName == NULL || *moduleName == '\0') {
 		Resume();
 		return;
 	}
@@ -134,7 +134,7 @@ ScreenSaverRunner::_LoadAddOn()
 			fprintf(stderr, "Unable to find the instantiation function.\n");
 		} else {
 			BMessage state;
-			fSettings.GetModuleState(fSettings.ModuleName(), &state);
+			fSettings.GetModuleState(moduleName, &state);
 			fSaver = instantiate(&state, fAddonImage);
 		}