[core] Fix crash on transitionFrameFn callback

This is a tricky one:

We are storing a lambda at the member variable `transitionFrameFn` and
capturing `this` at this lambda function.

At some point when running the callback, we set `transitionFrameFn` to
`nullptr`, effectively destroying the callback that is being executed.

After this point, `this` is no longer valid, as the memory gets
cleared and it might point to the correct location (or not). That
is why it works on some compilers but not on g++ 5.3.1. Touching
any `this->[smth]` is undefined after that.

The fix is just move `transitionFrameFn = nullptr` to the last
thing we do on the callback. This behaves similarly to calling
`delete this` on a class method.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/mbgl/map/transform.cpp b/src/mbgl/map/transform.cpp
index b38520b128..98b0804482 100644
--- a/src/mbgl/map/transform.cpp
+++ b/src/mbgl/map/transform.cpp
@@ -539,8 +539,11 @@ void Transform::startTransition(const CameraOptions& camera,
             view.notifyMapChange(MapChangeRegionIsChanging);
         } else {
             transitionFinishFn();
-            transitionFrameFn = nullptr;
             transitionFinishFn = nullptr;
+
+            // This callback gets destroyed here,
+            // we can only return after this point.
+            transitionFrameFn = nullptr;
         }
         return result;
     };