Auto merge of #13600 - slester:13586, r=jdm

Invert conditional and return early for AddEventListener/RemoveEventL…

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Cleans up code as described in #13586 by returning early instead of nesting everything in an `if` statement.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13586

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- [x] These changes do not require tests because the change is a cleanup of code and the author of the issue stated that new tests were not required.

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…istener

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components/script/dom/eventtarget.rs

@@ -515,7 +515,10 @@ impl EventTargetMethods for EventTarget {
                         ty: DOMString,
                         listener: Option<Rc<EventListener>>,
                         capture: bool) {
-        if let Some(listener) = listener {
+        let listener = match listener {
+            Some(l) => l,
+            None => return,
+        };
         let mut handlers = self.handlers.borrow_mut();
         let entry = match handlers.entry(Atom::from(ty)) {
             Occupied(entry) => entry.into_mut(),
@@ -531,14 +534,16 @@ impl EventTargetMethods for EventTarget {
             entry.push(new_entry);
         }
     }
-    }
 
     // https://dom.spec.whatwg.org/#dom-eventtarget-removeeventlistener
     fn RemoveEventListener(&self,
                            ty: DOMString,
                            listener: Option<Rc<EventListener>>,
                            capture: bool) {
-        if let Some(ref listener) = listener {
+        let ref listener = match listener {
+            Some(l) => l,
+            None => return,
+        };
         let mut handlers = self.handlers.borrow_mut();
         let entry = handlers.get_mut(&Atom::from(ty));
         for entry in entry {
@@ -552,7 +557,6 @@ impl EventTargetMethods for EventTarget {
             }
         }
     }
-    }
 
     // https://dom.spec.whatwg.org/#dom-eventtarget-dispatchevent
     fn DispatchEvent(&self, event: &Event) -> Fallible<bool> {