valgrind identified UMR in an example

https://travis-ci.org/jeking3/thread/jobs/457654740#L6689
```
testing.capture-output ../../bin.v2/libs/thread/test/ex_make_future_lib.test/clang-linux-6.0/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/ex_make_future_lib.run
====== BEGIN OUTPUT ======
==33300== Memcheck, a memory error detector
==33300== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==33300== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==33300== Command: ../../bin.v2/libs/thread/test/ex_make_future_lib.test/clang-linux-6.0/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/ex_make_future_lib
==33300==
../../libs/thread/test/../example/make_future.cpp 92
../../libs/thread/test/../example/make_future.cpp 98
../../libs/thread/test/../example/make_future.cpp 103
0
../../libs/thread/test/../example/make_future.cpp 108
==33300== Invalid read of size 4
==33300== at 0x405B19: main (make_future.cpp:110)
==33300== Address 0xffeffed4c is on thread 1's stack
==33300== 244 bytes below stack pointer
==33300==
```

I haven't tried it with the latest valgrind however. Current version is 3.14.0 and this was detected in 3.10.0.