Prevent caught error when using register(accelerator, callback) sig…

…nature

Before, when calling `register(accelerator, callback)`, while shuffling the argument variables to convert from the 2-parameter to the 3-parameter signature, `win` was left holding the accelerator string, and this was passed to `title()` which expects a BrowserWindow, generating an error:
    TypeError: win.getTitle is not a function
The error was caught in `title()`, which returned "A destroyed window", leading to an inaccurate debug log.

This was not a serious issue, but even caught errors become a nuisance when debugging with breaking on caught exceptions enabled, since you have to skip over them each time you restart the program, to get to the errors you're interested in.

index.js

@@ -156,6 +156,7 @@ function register(win, accelerator, callback) {
 		wc = ANY_WINDOW;
 		callback = accelerator;
 		accelerator = win;
+		win = null;
 	} else {
 		wc = win.webContents;
 	}
@@ -169,7 +170,7 @@ function register(win, accelerator, callback) {
 		return;
 	}
 
-	debug(`Registering callback for ${accelerator} on window ${title(win)}`);
+	debug(`Registering callback for ${accelerator} on ${win ? `window "${title(win)}"` : `all windows`}`);
 	_checkAccelerator(accelerator);
 
 	debug(`${accelerator} seems a valid shortcut sequence.`);