[ios, macos] Resolved unused variable warnings

Resolved unused variable warnings in Release configuration that occurred because they were only used within NSAssert() invocations, which are omitted from Release builds.

platform/darwin/src/MGLStyle.mm

@@ -341,10 +341,10 @@ static NSURL *MGLStyleURL_emerald;
         styleLayer = [[MGLCircleStyleLayer alloc] initWithIdentifier:identifier source:source];
     } else if (mbglLayer->is<mbgl::style::BackgroundLayer>()) {
         styleLayer = [[MGLBackgroundStyleLayer alloc] initWithIdentifier:identifier];
-    } else if (auto customLayer = mbglLayer->as<mbgl::style::CustomLayer>()) {
+    } else if (mbglLayer->is<mbgl::style::CustomLayer>()) {
         styleLayer = self.openGLLayers[identifier];
         if (styleLayer) {
-            NSAssert(styleLayer.rawLayer == customLayer, @"%@ wraps a CustomLayer that differs from the one associated with the underlying style.", styleLayer);
+            NSAssert(styleLayer.rawLayer == mbglLayer->as<mbgl::style::CustomLayer>(), @"%@ wraps a CustomLayer that differs from the one associated with the underlying style.", styleLayer);
             return styleLayer;
         }
         styleLayer = [[MGLOpenGLStyleLayer alloc] initWithIdentifier:identifier];

platform/macos/src/MGLMapView.mm

@@ -2419,9 +2419,11 @@ public:
 
 - (void)addOverlays:(NS_ARRAY_OF(id <MGLOverlay>) *)overlays
 {
+#if DEBUG
     for (id <MGLOverlay> overlay in overlays) {
         NSAssert([overlay conformsToProtocol:@protocol(MGLOverlay)], @"Overlay does not conform to MGLOverlay");
     }
+#endif
     [self addAnnotations:overlays];
 }
 
@@ -2430,9 +2432,11 @@ public:
 }
 
 - (void)removeOverlays:(NS_ARRAY_OF(id <MGLOverlay>) *)overlays {
+#if DEBUG
     for (id <MGLOverlay> overlay in overlays) {
         NSAssert([overlay conformsToProtocol:@protocol(MGLOverlay)], @"Overlay does not conform to MGLOverlay");
     }
+#endif
     [self removeAnnotations:overlays];
 }