Improves `elementAt` to be platform consistent

RxSwift/Observables/Implementations/ElementAt.swift

@@ -26,13 +26,24 @@ class ElementAtSink<SourceType, O: ObserverType where O.E == SourceType> : Sink<
         switch event {
         case .Next(_):
 
+            if i < 0 {
+                rxFatalError("index can't be negative")
+            }
+
             if (i == 0) {
                 observer?.on(event)
                 observer?.on(.Completed)
                 self.dispose()
             }
-
-            i--
+            
+            do {
+                try decrementChecked(&i)
+            } catch(let e) {
+                observer?.onError(e)
+                dispose()
+                return
+            }
+            
         case .Error(let e):
             observer?.on(.Error(e))
             self.dispose()

RxSwift/Observables/Observable+StandardSequenceOperators.swift

@@ -194,6 +194,6 @@ extension ObservableType {
     @warn_unused_result(message="http://git.io/rxs.uo")
     public func elementAt(index: Int)
         -> Observable<E> {
-            return ElementAt(source: self.asObservable(), index: index, throwOnEmpty: false)
+            return ElementAt(source: self.asObservable(), index: index, throwOnEmpty: true)
     }
 }

RxSwift/Rx.swift

@@ -37,6 +37,15 @@ func incrementChecked(inout i: Int) throws -> Int {
     return result
 }
 
+func decrementChecked(inout i: Int) throws -> Int {
+    if i == Int.min {
+        throw RxError.OverflowError
+    }
+    let result = i
+    i -= 1
+    return result
+}
+
 extension NSObject {
     func rx_synchronized<T>(@noescape action: () -> T) -> T {
         objc_sync_enter(self)

RxTests/RxSwiftTests/Tests/Observable+StandardSequenceOperatorsTest.swift

@@ -3708,7 +3708,7 @@ extension ObservableStandardSequenceOperators {
         }
         
         XCTAssertEqual(res.messages, [
-            completed(320)
+            error(320, RxError.ArgumentOutOfRange)
             ])
         
         XCTAssertEqual(xs.subscriptions, [