osfiber_ucontext: Fix memory leak & warning
osfiber_ucontext: Fix memory leak & warning

Do not move the `target` `std::function` on to the stack before invoking. The call may never return, and the `std::function` may never be destructed.
Initialize `Args` union - `b` may not be assigned on 32bit architectures, resulting in a warning.

google/marl,google/marl,google/marl,google/marl