Calling boolValue instead of converting NSNumber

Problem found with static analysis code of Xcode 9.3.

lottie-ios/Classes/AnimatableLayers/LOTCompositionContainer.m

@@ -38,7 +38,7 @@
     if (ENABLE_DEBUG_SHAPES) {
       [self.wrapperLayer addSublayer:DEBUG_Center];
     }
-    if (layer.startFrame) {
+    if ([layer.startFrame boolValue]) {
       _frameOffset = layer.startFrame;
     } else {
       _frameOffset = @0;