[core] Check if frameZoom is NaN in Transform::flyTo callback

src/mbgl/map/transform.cpp

@@ -293,6 +293,11 @@ void Transform::flyTo(const CameraOptions &camera, const AnimationOptions &anima
         Point<double> framePoint = util::interpolate(startPoint, endPoint, us);
         double frameZoom = startZoom + state.scaleZoom(1 / w(s));
 
+        // Zoom can be NaN if size is empty.
+        if (std::isnan(frameZoom)) {
+            frameZoom = zoom;
+        }
+
         // Convert to geographic coordinates and set the new viewpoint.
         LatLng frameLatLng = Projection::unproject(framePoint, startScale);
         state.setLatLngZoom(frameLatLng, frameZoom);

test/map/transform.test.cpp

@@ -36,6 +36,11 @@ TEST(Transform, InvalidZoom) {
     cameraOptions.center = LatLng { util::LATITUDE_MAX, util::LONGITUDE_MAX };
     cameraOptions.zoom = transform.getState().getMaxZoom();
 
+    // Executing flyTo with an empty size causes frameZoom to be NaN.
+    transform.flyTo(cameraOptions);
+    transform.updateTransitions(transform.getTransitionStart() + transform.getTransitionDuration());
+    ASSERT_DOUBLE_EQ(transform.getZoom(), transform.getState().getMaxZoom());
+
     // Executing flyTo with maximum zoom level to the same zoom level causes
     // frameZoom to be bigger than maximum zoom.
     transform.resize(Size { 100, 100 });