Fix crash when frames not preloaded in time.

GifuTests/GifuTests.swift

@@ -20,6 +20,12 @@ class GifuTests: XCTestCase {
     XCTAssertTrue(animator.isAnimatable)
   }
 
+  func testCurrentFrame() {
+    XCTAssertEqual(animator.currentFrameIndex, 0)
+    XCTAssertEqual(animator.currentFrameDuration, NSTimeInterval.infinity)
+    XCTAssertNil(animator.currentFrameImage)
+  }
+
   func testFramePreload() {
     let expectation = expectationWithDescription("frameDuration")
 

Source/Animator.swift

@@ -92,7 +92,7 @@ class Animator {
   /// - parameter index: The index of the duration.
   /// - returns: The duration of the given frame.
   func durationAtIndex(index: Int) -> NSTimeInterval {
-    return animatedFrames[index].duration
+	return animatedFrames[safe: index]?.duration ?? NSTimeInterval.infinity
   }
 
   /// Checks whether the frame should be changed and calls a handler with the results.